How can I improve my aim on Urban Terror?

Practice, practice, practice... use slow sensitivity and keep trying.

More hours playing against better players than you (easy way to see it's true: you kill nothing and dead always). Try playing 1vs1 against stronger players. Try to play clan wars with some friends. If your rival is better than you, even better for you.

Something to get into account is, more than hit the others, is not beeing hit. Here the key is: position, position, cover. Learn to have always a cover with you. And learn to dodge the other's shots (moving left and right while shooting).

Maybe it seems too hard to be just a game, and I don't know how much time you've been playing this, but think that usually the "pros" have been playing for more than 5 years.

Good luck & have fun :D

Try to shoot two or three shots at time, don't keep your finger on the mouse. Use the dot crosshair, don't run while you are shooting, and aim to the head.