You can't win every battle. If things get desperate, make an escape. Live to fight another day and all that jazz.
Keep moving! Practice until dodging is reduced to a reaction. If you take time to think about what to do, you'll end up dead.
Bunny hopping is like painting a big target on your face. Knock it off! Moronic purposeless jumping is a lame excuse for strategy. It only makes you stick out and rarely keeps you alive.
The biggest gun doesn't always win. I personally have a nasty practice of killing 133t rocket launcher commandos with the Enforcer. Master the use of each weapon, but also use what is appropriate to the situation. Try to develop tactics that involve several weapons, flush someone out with rockets or other explosives and then finish them with the flak cannon or pulse beam when they get close... you get the idea. UT's weapons are diverse for a reason.
Remember that rockets have splash damage. Don't try to make direct hits with rockets. Aim at surfaces you expect players to move towards. Keep a step ahead rather than on-the-fly. It's not a duke-out fight, it's a test of tactics.
LISTEN! If you hear your opponent coming, you can get into a position to ambush them. If you can place sounds within the UT world, you increase your awareness of your surroundings substantially.