Didn't have time to read all the posts, but here's what I have to say:
1. Have the fastest internet connection. If you're on a dial-up, you don't have a chance. That's just the way it is.
2. Have keys you can reach easily. Many people use W for forward movement, S for backward movement, A for left strafe, and D for right strafe (the so-called WASD config) but I can't reach those keys easilly, so I use A to move forward, Z to move backward, shift to strafe left, and X to strafe right. Same thing, just shifted down a row.
3. Circle strafe! Knowing how to do this somewhat complex move can be the difference between being at the top or the bottom of the scoreboard.
4. Practice! As you play against (and get your ass smeared by) better players, learn from their techniques. Of course, if you have a 150+ ping (see #1) no ammount of practice will help. I learned that the hard way.
Hope that helps. Of course, just about everything I siad was probably already said by other people.
