We all like to pretend that it's what's on the inside that matters most when we're picking a mate.
You want to pick a name that truly represents who you are and the kind of person you are looking to meet.
Dating expert, I recommend choosing an actual word rather than initials or letters. You want to avoid using your actual name unless you combine it with a descriptive quality. If you want to use numbers, then make them relevant to who you are or who you want to meet.
1) Pick a mature (not silly or arrogant) screen name.
Big Mikey9inch may sound clever to you, but chances are Sweet Jen28 will want nothing to do with a guy who feels the need to "advertise" (particularly when you're probably closer to Not So Big Mikey4inch).
Your screen name can be one that will make more sense as someone gets to know you.