First, a confession. This is a department where I personally need a lot of improvement. But I sometimes do manage to keep up with it, and I've given it a lot of thought, so hopefully I'll take some of my own advice.
When your living relies on constantly bringing in new projects, NOT burning bridges and keeping up with older contacts ensures that your circle of opportunity (to use buzz-speak) keeps growing, and not shrinking. Yes, there will invariably be a project that just goes so badly (alignment of the stars, miscommunication, whatever the reason) that you'd prefer to forget it ever happened. Aside from those, try to keep in touch with everyone you've done work for. Not aggressively so, don't make them cringe whenever they see your name pop up in an email. Just make sure that you stay on their radar as available. I wouldn't recommend contacting them more often than every few months.
How you keep in touch depends on the person and what kind of work you did for them. You may find social networking sufficient for making sure they don't forget you exist, or you may prefer on rare occasion to send them an update on what you've been up to (keep it short and professional) and ask if they have any open assignments they might consider you for.
An important thing to keep in mind is that in many industries (publishing, for example), the world gets small very fast. Behaving badly does get noticed, and can get around, biting you painfully when someone you were rude to moves to a publication or company that you've been trying to land. Just remember that when you're tempted to give someone the verbal finger.