The most important tip is to GO FOR THE OBJECTIVE. Don't be one of those idiots who worries about their personal stats. Either go for the objective yourself or help the people who are going for the objective. If you don't want to go for the objective, play a different FPS game. I'm serious.
Second most important tip: NEVER GIVE UP. If you keep getting owned, try a different approach. Figure out what the enemy team is doing that's owning you, and adjust your tactics accordingly. Just as important as going for the objective is removing the obstacles that the enemy team puts in your way. Also, remember that players leave, join, and get switched to the other team all the time. The opposing team may have all the good players now, but that doesn't mean that it won't be the other way around 5 minutes from now. Make every second count. I've been in many matches that have been won at the last second or in overtime. Attack fervently on offense/never let your guard down on defense until you hear the buzzer.
Third most important tip: COMMUNICATE WITH YOUR TEAM. If you have a headset or a microphone, this is ideal, if not, you can still type. Inform your team when you've taken out a key sentry gun or defender so they know that it's safe to rush. You'll be amazed at how quickly your team will coordinate just from someone saying "SGs down RUSH RUSH!" The same thing goes for listening to your teammates. You don't have to do everything someone suggests, but usually the more players you have trying to accomplish the same goal, the more success you'll have.
