This is how my AIM buddy list looks like

Four categories:
- People I talk to
- People I talk to sometimes
- People I don't talk to
- People I don't remember

I keep "People I don't remember" in case I do remember them.

The common alternative is to make a folder for different affiliations (i.e. high school, work, college, etc..). Usually, though, this doesn't take advantage of the drilldown menu structure of buddy list.

I usually keep the "people I talk to" open most of the time. And because this isn't many people, it doesn't occupy much space. Also, I frequently slide people in and out of groups.