Some of them will lose interest. Move on. I have 2 FB's I can call pretty much anytime, and they don't consider themselves my GF's, which makes it easy. I have one more whom I can call occasionally for a hangout / lay but I think she's getting serious with her AFC BF right now so that's probably on the outs at least temporarily, a chick I just met this weekend who hasn't been categorized yet -- not sure if she's an interesting enough lay to warrant a second round -- and 4-5 more in a holding pattern via email, myspace, & phone / text.
I keep an Excel spreadsheet on the entire stable on my laptop. How we met, where we went and when, her hobbies if any (amazing how many girls have no hobbies!) what jokes I told, their sexual preferences and hangups, birth control method if applicable, menstrual cycle, etc. I consult it before every date. Serious.
The only time it gets complicated is when a girl calls and doesn't tell me who it is. Then I have to let her talk until I get it figured out. Often, that involves cross-referencing the database, so I'll bust out my laptop.
"What's that noise? Are you typing?"
"Just a little work. Sorry, keep going. . . um, Janine. . . ."