1) if you think someone is around the corner, or a sniper is watching the corner, let your teammate go first
2) use a Sub-Machine gun most maps, (best IMO is the Mini-Uzi with a holographic sight)
3) comando pro, nuff said
4) if you can stand using a handgun as your secondary, tact knife goes a long way, otherwise use one acording to the map, i.e. if its a map like skidrow or favela with alot of ally ways, a shotgun can go a long way, if its something like high rise or wastland where alot of the combat is cross map or holding an area, having a machine pistol is pretty usfull, run through the ammo on your primary then switch over
5)assume the other team is going to camp, and rush accordingly
6)if your on sub base, throw a flashbang or stun grenade through the window before you go up the stairs to the server rooms (you'll figure out what i mean)
7) if your going to camp, learn how to do it without looking retarded, move a bit, be in prone when you can, and know where people can come from
8) knifing runs will piss off the other team, and they will change strategy to fit them
9) Turrets can make great use for holding a position, setting them pointed the other direction of you, and letting them cover the other entrance, you can also set them as traps, waiting near by in the dark for someone to nock out a poorly placed turret, then sneak in and knife, also work for pulling people to an area, keep a turret live and go to a sniping point a ways away, snipe off people when the turet fires near them, or they get too close
10) if your going to use thermal, at least have another class set up for snow filled maps that make thermal useless or at least inneffective
11) Have a sub, marathon pro, lightweight pro, comando pro class set up, they work well on close quarter maps
12) if you can do it right, going down into prone while your shootoing at someone can give you an extra second at them, wich depending on your weapon can take them out