setting up a simple tickler file to help keep track of your network. It's a simple but awesome tool that you can tweak to fit your specific needs. Once you have one, though, the question arises: When do you remove someone from the list?
For me, the two main reason I would remove a networking contact are if they either, one, didn't value me and my reputation, or, two, if they devalue my reputation.
Not valuing: This behavior is best exemplified through a lack of willingness to stay in contact. If I've made an effort to stay in touch with someone, using a variety of mechanisms (email, phone, written note, etc) and I never hear back from them, I can only assume that they don't value me. Now, in this case, I don't make snap judgments. Just because someone doesn't return my call doesn't mean I'm going to write them off. This has to be behavior that I witness over the course of a year or more. If it ends up, though, feeling like all of my communications are slipping down a black hole, never to be seen nor heard from again, it behooves me to put my efforts into those relationships where I am valued.
Devaluing: This is a much more serious breech of the relationship. If I vouch for someone to a friend or colleague, they have to know that I am lending them my reputation. If the referral goes south, then it makes me look bad. In that case, I will try to work with the two parties to help get things resolved. If it should happen multiple times, or if the person I refer is unwilling to make some effort to clear up the problem, then they've damaged more than their own reputation. In that case, the only thing you can do is pull back from the contact and wish them well in their future pursuits.
Truthfully, you might have a variety of other reasons to remove a person from your regular contact schedule. The main caveat is: Never rush to judgment. So long as they aren't damaging your reputation, giving them the benefit of the doubt probably won't hurt you. After all you've already spent a fair amount of time cultivating the relationship. Make sure you aren't throwing that investment away without good reason.