Try the pomodoro technique (its just a 25-minute timer). I only tried it quite recently, but its working great and quickly becoming a habit. I really appreciate the no-nonsense approach which prevents any yak-shaving[1]. Its simple on the most basic level so its compatibile with more the complex systems like GTD, or task management / team collab apps.

1. http://en.wiktionary.org/wiki/yak_shaving