Here are some tips according to the latest version of counter-strike (1.4):

1. Try to avoid jumping when being shot at.  In earlier versions this was useful but now with the walking after the jump it just makes you an easier target.

2.  If you are on 56k or lower, try to aim a little ahead of where your target is.  Not very far in front of them but enough to give your bullets a bit of time to reach their target.

3People with low ping tend to rush more than camp.  Need I say more?

4. The Mp5 is the best gun for its cost.  Second round this should be your number one choice.  If you win and have the $3900 some people like to go for the big flashy guns like the Colt or the Ak.  The Ak is a good choice because it is really cheap, but unless your confident you can stay alive and not lose your new shiny Colt before taking someone out stick with the Mp5.

both pictures were taken from gamers.com