Never, and I mean never, stop moving. Always move, strafe, and learn to circle around people.
Crowds are fun.
Find one and lob a few grenades, rockets, or BFG blasts into it for some massive frags. You always have a better chance of winning a duel if it is fought in close quarters. If you are chasing or facing off against someone, always try to close the gap and get closer. Try to vary your attacks. Don't follow a pattern that people can easily learn and adapt to.