Teamwork is the heart of Counter-Strike. The "Every Man For Himself" deal only goes so far. Check out these tips...
Listen - Listen to your teammates. If they call for backup, there's usually a reason for it. Many teams lose becuase they don't back eachother up.
Talk - Use a microphone if you have it, or bind certain keys to teamsay messages. The radio commands can also be very useful. When used, they also flash your position on the radar.
Where? - Know where your teammates are. Watch the radar, watch to see if they die. If you know someone was in a certain area by himself, and he dies, its obvious that the other team is there.
Gun selection - Ask your teammate what equipment they're buying so you can
buy equipment that will compliment their's. This can be very important if you
are low on cash.