Fortifications:
Tyranid Fortification choices are severely constrained by their inability to use any of the guns that come with them. The Tyranid FAQ specifically prevents them from manually firing any of the emplacements, leaving the Aegis line without any offensive capability, and the Imperial Bastion and Fortress of Redemption able to only autofire at the nearest enemy unit at reduced BS.
This takes the Fortress right out of contention, as so much of its substantial point cost is built into its weapons. The Bastion is a different case. Significantly cheaper, it provides a good place to hide infantry units, and prevents them from taking Instinctive Behavior tests. It provides an AV 14 wall to hide behind, something that the Tyranids otherwise have no access to and that some armies may not have the tools to effectively counter. The downside is that Tyranid units have very few long range weapons that can be fired from within the Bastion, effectively taking whatever is placed inside out of the game. Even worse, if your opponent does have the means to breach the Bastion's armor, the damage results will devastate the unit inside. The possibility of a template getting near one of the Bastion's portals is also frightening given the poor saves in the Tyranid list, but isn't much worse than them being outside.
The Aegis Defense Line offers something different. The cheapest of the Fortifications, it allows a decent cover save for a large swathe of the battlefield. This offers great protection for most Tyranid infantry. Placing one of these near an objective can make the unit holding it extremely difficult to remove at range. There are two downsides to the Aegis line; the Tyranids cannot use either of the gun upgrades for the Aegis line, and have difficulty gaining the cover bonus for going to ground (since going to ground is impossible while in Synapse range). Of all of the Fortifications this one is the most useful to the Tyranids, although not essential. Its advantages are basically those of a piece of Ruins terrain; the main bonus is placement.
The Skyshield Landing Pad offers something that the Tyranids sorely lack: a decent invulnerable save at range. While it's ability to negate scatter isn't something that most Tyranid units need, the invulnerable save is something that the army can't get otherwise. The only downside is that most of the units that would benefit from the Skyshield will want to move off it, sooner rather than later. If your army contains a significant firebase it might be worth it; otherwise, probably not. Nevertheless, it offers something that you otherwise won't have access to.
Finally, the Comms Relay upgrade that can be bought with several of the above Fortifications is something to consider. Allowing you to reroll reserves, it helps ensure that your units arrive exactly when you need them. While it doesn't come cheap, if you plan to use reserves regularly and are already taking a Fortification, the Relay will be be useful.
Terrain
This phase can be fairly complicated or relatively simple. If your area doesn't use the rulebook setup, then this won't help much. If they are, then here are a few strategies I've found to be helpful. Large midfield terrain is your friend. While it will grant cover to both sides, most of your shooting won't be ignoring your target's saves anyway, while the same won't be true for your opponent's fire. Monstrous creatures and large broods are much harder to bring down when they have cover saves. Large midfield terrain also means that units that love to sit back and shoot will either have to move forward or have their effectiveness reduced.
Especially if you're playing for objectives, try to set up the terrain in a manner that will force your opponent to advance to get the full benefit of his offensive power. If that's not possible, try to ensure that there is terrain your units can leapfrog too as they move forward. Larger pieces are optimal, but the tactic will work with anything that grants a cover save.
Be aware the above strategy can also hurt you in assault. Tyranids have almost no assault grenades, and thus are adversely effected when charging into any kind of terrain. There's not much that can be done to mitigate this. Pinning a unit or forcing it to go to ground can help, but the former is unreliable at best, and the latter unlikely given the relative saves of most targets.
On offense and defense, the idea is to limit incoming fire while ensuring that you have a clear path to assault once the distance has been closed. This applies particularly in relation to setting up objectives, as these are the easiest means of determining where your opponent will go.
Psychic Powers
Psychic abilities are one of the strengths of the Tyranid army. In addition to their basic powers, they have access to Biomancy, Telepathy, and Telekinesis. All of these options can have a powerful influence on the course of the game. While Tyranid psykers are limited to Master Level 1, with the exception of the Swarmlord, they make up for it in volume. One caveat: while you can pick from multiple book disciplines, you cannot mix and match them with your codex powers. All three disciplines can significantly benefit a Tyranid swarm. I personally prefer Biomancy over the others, but each power can change a unit's battlefield role; let that guide your choices rather than what is "objectively" the best power.
Biomancy
This is the strongest discipline for Tyranids, particularly for our monstrous creatures. It has two powers that greatly reduce Tyranid vulnerability to heavy weapons: Iron Arm and Endurance. Iron Arm alone can make an MC almost impossible to kill at range, while Endurance does much the same, with the possibility of protecting more numerous units as well. Enfeeble can also make normally resilient units vulnerable to massed Tyranid attacks or shots, or make the already average truly fragile. Lifeleech can be handy for emergency healing or stripping the odd hull point from vehicles. Warp Speed can give much needed extra attacks in assault while ensuring that a model strikes at least on par with its opponents. Haemorrhage is probably the weakest power, but it at least holds out the possibility of sniping a troublesome weapon. The Primaris Power, while not particularly impressive, also offers massed AP 2 shooting, which may be desirable against some foes. Taking it is situational, but it provides and option the army otherwise doesn't have.
Telekinesis
This discipline is less useful, but still handy. On an MC, it should probably be chosen only after rolling Endurance or Iron Arm on the Biomancy table, although Zoanthropes can use it freely. Vortex of Doom will almost never be used, as it is essentially a more dangerous Warp Lance, and the only model that can use it has better things to cast. Crush, Assail, and Shockwave are all fairly lackluster. While each has potential uses, none of them offer anything that normal Tyranid shooting couldn't accomplish. The beam and Strikedown abilities of Assail make it situationally useful, but in most cases you'd be better off firing whatever weapons the psyker had. The same applies to Crush. Gate of Infinity, Telekine Dome, and Objuration Mechanicum all have potential uses. Gate can allow an otherwise slow unit to move across the board rapidly. Dome can give an extra layer of protection to a unit when cover is unavailable. Objuration can blunt incoming fire, and potentially damage vehicles at range.
Telepathy
This discipline is about on par with Telekinesis, with a greater emphasis on disruption. It's also slightly more predictable than the others due to the inclusion of 2 Warp Charge 2 powers that are useless on all but the Swarmlord. Mental Fortitude won't see much use, although it can function as emergency Synapse in a pinch. Dominate can potentially take a key unit out of the game for a turn. Terrify is unique in that it can strip away Fearless, rendering otherwise immovable units vulnerable to morale and leadership tests. Puppet Master is handy is particularly useful for turning an opponent's own firepower against them. The Primaris Power, Psychic Shriek, is handy in that it can ignore armor and cover saves, but whether you'll want it or not will depend on the role of the psyker. It is potentially devastating, but won't be consistent.
Hive Tyrant Codex Powers: The only one of these really worth discussing is Paroxysm. Although limited by range, this power can cripple a unit offensively and defensively by reducing its WS and BS to 1. While powerful, the possibility of getting other powers is generally going to be more useful throughout the game. Paroxysm is useful once you've closed in, but not so much in the early stages.
Tervigon Codex Powers: These present more of a choice. While a Tervigon can get up to three rolls in a given discipline, all three of its normal powers have great utility. Catalyst can provide Feel No Pain to threatened units, Dominion can boost Synapse range at critical junctures, and Onslaught can provide an extra few inches of range to a brood's shooting. Swapping any of these powers it is situational. While some of the discipline powers are outright better than the codex powers, others are either inferior or can change a Tervigon's battlefield role. Generally I prefer to play it safe and keep a Tervigon's codex powers, but some decent rolls on any of the charts can make them extremely dangerous for your opponent.
Zoanthrope Codex Powers: Zoanthrope psychic power selection comes down to the role you foresee for them in a given game. While their default powers make them ideal for hunting vehicles and heavy infantry, they can become very effective buffers/debuffers or distraction units with the right powers. The main downfall in the latter strategy is not knowing which abilities will ultimately be rolled.
Broodlord Codex Powers: The Broodlord is probably the only Tyranid psyker who benefits more from keeping his own powers than swapping them out. This is mainly due to his BS of 0, which prevents him from using any of the witchfire powers he might roll up - this includes powers that don't require a roll to hit at all, or that would otherwise hit automatically. It helps that his Hypnotic Gaze power got even better with the addition of challenges; in addition to preventing a character from striking, he now has a good chance to single them out and kill them without taking any damage. Some of the available psychic powers provide greater advantages, but none of them change what the Broodlord and his unit will be used for, and trying for them carries a good chance of getting powers that are absolutely useless.
Warlord Traits
Since Tyranids don't have unique traits, you'll be choosing from the 3 rulebook categories: Command, Personal, and Strategic. Of these, I recommend Strategic. While its utility will vary greatly with your list setup, it has the least number of options that will be useless. Conqueror of Cities can significantly reduce incoming shooting damage, while Master of Ambush, Strategic Genius, and Divide to Conquer will benefit swarms that make use. of reserves. Even Princeps of Deceit allows you to react to your opponent's deployment. Finally, Night Attacker can make Tyranids much more resistant to first turn shooting. Personal Traits give some potential durability and utility to your Warlord, but may or may not be useful; Legendary Fighter, Tenacity, and Immovable Object can potentially allow a Warlord to play a much greater role on the battlefield. Command Traits are probably the weakest of the three choices. Coordinated assault will be useful if your WArlord happens to be a combat brute, but the others provide minimal benefit; Intimidating Presence may be useful against lower leadership armies, but only if you can get close enough.
