Just a few tips and techniques I've picked up. Some very obvious, some not so obvious:
1) use jeep/buggy as mobile bomb by planting c4 all over it, jump out at the last second and let it ride itself into enemy area, detonate. same can be done with suicidal chopper missions.
2) if a medic near you dies (on your team or not), equip off him, revive him with his own paddles, drop a health pack and cover him.
3) if you're a recon the very first thing you should be doing when you spawn on the front line is throw a motion sensor out. that way if you die right away at least you've helped your team mates out
4) as above if you are assault (drop an ammo box ASAP)
5) with UAV, fire off a missile and back the hell off to your spawn under cover of trees etc to allow it to recharge. or, fly immediately behind the enemy and use it as a recon/spotting device, spotting as many enemies as you can for your team.
6) don't place mines near your own tanks... or mcoms. or your own buildings for that matter.
7) don't be so obsessed with getting the kill. if you can knock a few health points of some enemies then run for cover and chill for a bit you'll get assist points and stay alive longer. let your teammates finish enemies off while you resupply or get health.
8) think about your spawn position. it may be a better idea to spawn a little further back or off to the side rather than finding the squad member furthest up. he may well be in the middle of a ****storm and you don't want to dump into the middle of one of those...
9) change your class type as and when required. no single game ever calls for ONE class type the entire round. got a tank up front that's taking fire? next time you die switch to engy and go help the driver out by repairing his tank. you can switch back to recon/assault/medic again later.
10) same with medic. towards the end of a rush round, if your team is struggling switch to medic and start reviving as many as you can to give your team the best shot at winning.
11) running out of tickets in a rush round? dont forget you can destroy the mcom by shooting at it and planting C4/mines etc. go engy or recon and try to blow it up if you're running out of time and can't get in close enough to plant a fuse.
12) think before taking out enemy mines. make sure there are no teammates near by before blowing them up.
13) try to get behind the enemy (if the server allows it... which it should, because it's a perfectly valid tactic!). especially goes for UAVs and tanks.
14) engineers - use your tracers! nothing worse than a HIND or tank being allowed to roam the map for 15 mins taking your whole team out. trace that bitch and then take it out! no reason a tank or chopper should survive longer than 30 seconds if you know how to take them out.
15) if the server allows it (again, it should), don't automatically destroy an enemy tank if it's been abandoned by its driver and you've killed him. take the tank and drive it back to your base. or take it out. but don't leave it there for the enemy to use again. so don't repair it until you're back in your base. same goes for choppers. either steal it and take it back to base or plant C4/mines on it and leg it. then when the enemy goes to fly it... do your thing.
16) recons - your Q finger should be bleeding by the end of a round. if you're not spotting everything you possibly can all the time then you ought not to be playing recon. and don't forget, you don't need to physically see an enemy and THEN spot it - when you're creeping around just mash the Q key and occasionally you'll reveal an enemy you didn't even know was there. this is particularly useful when the map is full of smoke and debris.
That's it for now. If I think of any more I'll add to this list.
Cheers,