Lesson 1: Battlefield Scavenger
So you just started out, and you're determined to get some kills with one of the entry-level weapons so you can unlock, say, a bayonet or sniper scope. Well, don't let that keep you from swapping that pea-shooting pistol for whatever your fallen foes leave behind. Early in your career, what you find on the ground will almost always be better than what you can equip from the start, so scoop that gear up whenever you get a chance, even if just to get a feel for what upgrades you really want to work towards.
Lesson 2: Close Quarters
Want to own the game when things get tight? It takes some work to pimp out that M1897 shotgun with a bayonet as soon as possible, but you'll have a melee reach advantage if you miss, which could well save you from taking it in the neck if you get caught reloading. Once you hit level 29 and unlock the double-barreled beast, you should be practiced enough to unlock those add-ons faster.
Lesson 3: The Long Ranger
Generally speaking, you should always tailor your custom classes for a specific strength, and play those classes only on maps suited to them. For maps with clear views to distant terrain, that means outfitting a sniper with two important perks. Deep impact will let you take out goons that hide behind flimsy surfaces to reload, while stopping power will let you focus on center of mass if you've yet to master the art of the headshot.
Lesson 4: Hit or Miss
Some people are natural marksmen. For the rest of us, thoughtful selection of perks and weapon attachments can make all the difference. Load up an MP40 with dual magazines, and you can fire for a good long time before reloading. Add in the stopping power and steady aim perks, and a blind man could do some serious damage.
Lesson 5: ...and Stay Dead!
Sometimes you'll do enough damage to drop a guy, but not actually kill him. Get into the habit of pumping a couple rounds into bodies when they fall, and you'll find you don't get surprised by a bullet to the face quite as often. This goes double for any mode that involves capturing flags or control points, since even a crippled opponent can continue to capture a resource until properly wasted.