My long list of tips:
1. This tip is #1 for a reason. Always spot by aiming at an enemy (infantry or vehicle) and press Q, so everyone can see it. Regardless of kit, this is one of the best and simplest ways to help your team.
2. ALWAYS take cover. Run from wall to wall, concrete block to concrete block, crouching often and looking out slowly. Dont go Rambo and run out into the street and unload on some guy a ways away, cause you'll get blown to pieces =). You should rarely be out in the open unless you are really far from any enemies (but even then, enemy jets might mow you down while they are lining up an attack run).
3. As you unlock new things, mess around with certain kits and loadouts til you find one that you really like/works well. Makes the game more fun and brings you in some points.
4. Work with your team and communicate. If you see guys dying all around, and you're a medic, revive them (unless they're in the middle of the street in the middle of a firefight lol). If you're support, throw down an ammo box near a group of friends. If you see a tank that no one is taking out, switch to an engineer and take it out yourself. If you're a sniper, spot some guys from afar that your teammates may not see (like behind stuff). These tactics will bring in tons of points, as well as help your teammates, which can result in more victories.
5. Find some good vantage points and get some kills while you can, but don't stay there too long. With kill-cam, angry enemies will come searching for you, so move after a couple kills.
6. LISTEN! This helps me out so much! If you are running around and are afraid enemies are near, throw a grenade where you think they might be. They'll scream something ("Grenade!" for the US, "Grenata!" in Russian). This will tell you where they are (and it is direction sensitive, so if you hear them to the right, they are to the right), and will allow you to plan out your attack and kill them. Enemies also shout things when they throw grenades, when friendlies die, when they get kills, or when they throw down health or ammo boxes.
7. If you are really struggling, one thing I do is join a squad with someone who is doing pretty well, and stick to them. Switch to a kit they're not (like if they're a support/engineer, switch to medic and heal em; if they're medic, switch to support and resupply them) and follow them around, help them out, and learn from what they do. Chances are, they're team players and will help you out when you need it too.
8. The recoil is much more realistic in BF3 than any other FPS I've played. You should be able to compensate for it/keep it under control after just a little bit, but understand that you usually can't go full auto on someone more than 20 yards away.
9. Learn to use the environment to your advantage. Put mines in shrubs/grass on the road so tanks can't see them as well. Stay crouched/prone behind solid objects as much as you can, and use shadows to your advantage. People whine about camping, but another word for camping is defending.
11. If your team is doing very well, feel free to practice using vehicles. I won't say not to use vehicles when your team isn't doing well, but if you aren't confident in your vehicle-piloting/driving abilities, I'd suggest leaving them to the people who are. And by vehicles I mean tanks/helis/APC's/jets. Humvee's and such are always a good choice, but don't forget to wait for your teammates to hop in.
12. Most importantly, KDR is irrelevant. The only thing that matters in BF is winning, so play to win, not to get the most kills a.k.a "Playing the Objective". You'll inevitably run into 'those guys' that'll run around with an acog/dot on their sniper and go 20/3. Yeah, they get lots of kills, but while they're doing that, their team is losing all their bases/getting their MCOMs blown up.
I hope this helps, and I hope to see you out on the Battlefield soon =)
*Language Warning*
This video is back from BC2, but a lot of the information holds true, plus it's hilarious
[ame]http://www.youtube.com/watch?v=0p4fkH9qC4M&ob=av3e[/ame]
I still don't know why its so hard to reduce splash damage and keep direct the same(i.e. piercing damage) for rockets as from what I understood they were designed more to pierce enemy armor than be an absurdly powerful explosion.
Learning the maps is a great deal of doing well, knowing where to go and when. Practice practice practice. 
