Maybe it's not a science, but I needed to share a few things with the community, because I don't think most guys get it. Stop ending your casual relationships. So many guys, when they get into a relationship, or go away for the summer, or you just haven't seen your fuck buddy in a month, either explicitly or imply that it's over. If you both just stopped texting, the door is still open quite literally at any point. It's not weird, it's not out of blue. Stop putting yourself in a box of "societal norms". Real men ask for what they want.

Secondly, if you do get exclusive with a girl, let your fuck buddies know that you value them for what they are, and leave the door open. I sent this exact text to a new casual relationship when I knew I had to commit to a great girl. "I think you're really cool. Bad timing unfortunately, things have progressed a little faster than expected with a girl I've been seeing. I need to give it a shot. I had a good time with you. You're fun to hang out with and painfully beautiful, and I still get turned on when I think of making you shake ;). If things don't work out with this girl, and you're available, hopefully we can pick things back up. And yeah, we're at different places in our lives, but that shouldn't stop us from sharing some amazing sex."

I gave her value by telling her that she is cool, fun, and sexy. Everyone wants to feel that way about themselves. Moreover, they want to be around people that make themselves feel that way.

5 months later I broke it off with my g/f and texted "Jenny.. I'm single and would like to see you. You wanna grab a drink this week?"

Be a man. Be authentic. Assume attraction with every girl you're into, and let them know you find them desirable. Women desire to be desired, and like being around men who make them feel desirable. Don't string along any girls who want you to be their b/f but you only want to fuck. Girls like sex too, and you don't have to date everyone you sleep with.

tl;dr. Add new ones when you can, but don't cut ties with the old ones! They're just a friend you haven't seen in awhile.