Hahaha, do the first thing which comes to your head. When I was joining, I went with "Rubber Nipples" because I thought it reflects my personality well, wherein I'm into a lot of obscure shit... Also, I like that Ren and Stimpy Episode.
Other names I was going to name myself was:
Salmon Police, Hanging Vice, Salt&PepperLonelyheartsclubband, ChickenTits, RubberCollarDisco, PurpleMonkeyDishwasher...
I'm unsure if a lot of those are taken (probably) but I just follow whatever my mind is into.
Perhaps look at various thread and what of other usernames you like and if they have something in common? I for example have found I tend to like short names, that I can associate a thousand things to semi-secretly, which have colours I relate too. I like two-syllable names often too, if three, it has to have a nice rythm to it. You probably have other criteria, but perhaps if going trough a bunch of other usernames you could see some pattern for what you personally like like that, say three criteria, and then make up one from those.