A favourite topic of mine.
I like to keep my FBs (actually I only have 1 atm goddamit!!! coz I didn't heed the advice I am about to give) wanting more. If you are too available they lose a bit of excitement and they will move on (or fall; see the next point).
I rarely initiate sex or make booty calls and leave that in the girls court. Generally even FBs get a bit shy about intiating but they will give you strong signals when they want it. This means that they will give it up on those rare occasions you NEED some lovin. More significantly it easy for girls to become attached. Sex is a pretty emotional thing and if a girl is experiencing (great) sex with you often her feelings will start moving towards commitment. Furthermore if YOU are initiating often she will feel that you "owe" her or start believing that you are too have stronger feelings.
Be her friend first and foremost. The quality of both the sex and the rest of your interactions will be so much more fulfilling.
Don't be secretive about your other girls.
If they start getting funny then it is a sign they are falling. Back off or you'll have a teary girl to deal with.
NEVER get jealous of other guys. Infact encourage it. This has two effects.
1. It reminds them that they don't own you and that they can't say **** if you are with other chicks (done well and they will act as a pivot).
2. For some reason if you encourage a girl to hook up with a bloke or show pleasure that they picked up they will see you as a better man than the other guy.
Encourage her to experiment with lesbianism (I think the reason for this is obvious... but I'll be honest here, I have not yet reaped the benefits of this)
Don't fall into the trap of treating her contemptuosly (its easy to do!). It won't necessarily make them leave but it will mess with their self esteem.
And never make them feel like a ***** or they are gone (except when they want to be made to feel like a ***** of course).
JJ