Then it's a case of picking either Stasis - the level six bubble is especially cool - to pin enemies in place, or the Lift Grenade, for emergency crowd control. Another a cool build is the Salarian Engineer. You're most effective hanging back in 'midfield', using Energy Drain to strip shields and then sniping. (Try taking the Incisor sniper, as an alternative to an assault rifle.)
The Decoy ability is especially handy for taking the heat off your team - even Banshees will zero in on your Total Recall-style projection, allowing you to either counter attack or make a heroic retreat. As you get a feel for a character's abilities, you'll get a better sense of how you want to play them. At that point you might consider using one of the bonus 'power resets' you're likely to have picked up buying equipment packs to refit the character's powers around your tactics. Alternatively, by switching gender or race in the character select screen you can create another character in the same class, with all the points you've earned carried over.
One Gun = More Fun
Unless you're playing the Soldier class, you really don't need to take more than one weapon into battle. Vanguards, Sentinels and Adepts are all especially dependant on using their powers, so having a quick recharge time is essential - and as you'll know from single-player, the more guns you're carrying, the slower your power recharge is. Likewise, even with a one-gun loadout, keep an eye on the weight of that weapon. The vanilla guns might seem dull, but they're light, and once upgraded to the X version, can still pack a meaty punch.
Anyway, weapon-swapping in a firefight wastes valuable seconds. So long as you've got the discipline to reload at every opportunity, and remember to stock up on ammo from the boxes between waves, you won't need a back-up piece. Get it right, and a Human Sentinel will be able to spam Throw and Warp, then use an assault rifle to mop up any remaining chunks of energy. It's remarkably effective, and with the added bonus of Tech Armor to keep you safe, a very versatile character.
"Turian, Heal Thyself!"
The only time it's really worth rubbing on that scarce Medi-Gel is if you think returning to the battlefield is going to save the wave for your team. Certainly, if you've got teammates nearby (and they're likely to rescue you because they don't seem like douches), or the round is almost over anyway, then it's not worth wasting a Medi-Gel - especially in the earlier waves.
The exception is in the final few waves, when it's worth doing almost anything it takes to ensure you get those completion bonuses. Spec Ops packs, which instantly restore health and shields, seem to be more plentiful and should accordingly be used more freely. Especially if you're on the other side of the map, and it'll disrupt your buddies to have to slog over to save you. But saying that...
No-one Likes A Soloist
Before long, you'll have come first a couple of times, and start wanting to go all Kenny Krogan, storming around the map hoovering up XP. Don't. Even on Bronze, it's an annoyance having to schlep over to the other side of the map to resurrect your ass. And you will die. On later waves, and especially higher difficulties, working together is essential. Don't worry, over the course of the round you'll still rack up plenty of XP, especially as assists are worth more points dependant on how much damage you've done to a target.
Catching more powerful enemies in crossfires is the quickest way to take them down, and objective-based waves are much easier to tackle with wingmen alongside you. Oh, and if you must be an XP whore, for god's sake don't forget to get on the shuttle before the clock runs down, thereby costing your teammates the valuable 'full extraction' bonus. Or you can expect a very frosty reception/kicking in the lobby.