So as a Blood Angel player, I am very fond of Jump Infantry. Get to assault ASAP. That is what I do.
Most players may have only one, if even that, squads of Jump Infantry. This does not include Tau Battlesuits, as they work differently.
Now there are several primary uses for Jump Troops: Attack, Distract, Annoy, and React.
The first of these is what I use in my Blood Angels. This is where Jump Troops form your tip of the spear and drive straight into the enemy. Best used en masse. You take the fight to him as fast as possible, utilizing cover and arriving in assault by turn 2 (3 at the latest). You take assault weapons or plasma pistols if you can, and power weapons are a must when available. You are ripping your opponents throat out and denying him a chance to capture objectives, or just plain slaughtering for the fun of it. If you can take out our opponent's troop choices that is a definite perk. Make sure though you have 1 or 2 slow squads following up, giving support fire, engaging tanks, and grabbing objectives in the wake of the destruction. Jet Bikes/Bike also fit in nicely to this form of army, sprinting forward in line with the Jump Troops.
Distract is another use. Now this may seem like a bad idea to most people, but I have used it many a time to great success with terminators and death company. The idea is you place a very menacing high point squad out where they can draw off the most enemy units. Let them get shot at. Let them be noticed. Expect to lose them to a man. Jump Troops can perform this role nicely, as they get close fast, and typically players react to the most immediate threat. For this, you want something that looks mean in the squad. A power fist or melta weapons are good examples, especially if it looks like you are going to your opponents tanks. And if worst comes to worst, they ignore the squad and concentrate on the rest of your forces following up. In that case they have made a mistake and will regret it once your Jump Infantry falls onto their heavy support (which should be done in this case to neutralize their firebase).
Next is Annoy. This is where you send your Jump Infantry up to do pesky things. Pick off Tyranid Warriors. Reveal hidden models. Engage with stealth units. Harass heavy support. Basically annoy is a catch all for everything not in the other categories. This is used the most by people, as typically someone has no more than one or two squads. Niche use mission to mission is the key here. Contesting objectives last turn also falls in here, and is great use of Jump Infantry, as they can be a ways away and show up close enough to deny your enemy victory.
Lastly in React. This is where you let your main force do most of the work and Jump Infantry sit back and jump in where needed. About to loose an objective? No, because your assault squad has come in and scattered your opponent. Your troops tied up in an assault. No longer as your Warp Spiders rush up and lend their numbers. Devastator squad got you pinned from the top of a building? Not once those Vespids are up there blasting them apart. Basically, you hold your fastest troops back to bulk up your line where it is weakest. In addition, any deepstriking troops can be dealt with by a reaction force that has been hanging back.
Another note is never Deep Strike Jump Infantry. They can assault by turn two and reserves come in turn 2 at the earliest and can't assault. So it is too slow. AND it leaves what is most likely a hard hitting force sitting for a turn clumped together as a nice target for shooting or assault.
Another thing is that often Jump Infantry squads are outnumbered. Be wary of this, make sure you get the assault to bring the number of attacks up. Do as much damage as possible. Jump Infantry don't have to survive, they just have to destroy.
