Team Dignitas posted a guide on how to improve your aim. Check it out here.
Death match practice with a goal of X amount of headshots works pretty well. Usually we do a few practice rounds before getting into competitive match making. It helps because you can focus on getting lots of kills or focus on just your aim. Training only your AK or M4 skills is great to do in death match.
They also recommend doing offline with bots mode, with the bots set to harmless/easy. Bots running around can help you aim at a moving target, like the double doors with an AWP on de_dust2.