Nyu, nicknames...I didn't bother with them before, but nowadays I name each one I catch (Except legendaries) out of principle. It's fun, and besides, it's good practice since my RP characters always name their pokémon anyway. xD
Anyway, my nicknames are all over the place. Sometimes I pick them because they sound right (E.g. Kula the Snorunt and Baku the Camerupt), some of them are bastardized versions of the japanese names (Can't think of any from the top of my head, though xP), some are descriptive (E.g. Fanfare for my Phanpy), some just names that I thought sounded nice (like Minerva the Vaporeon), and some are just plain random (Like Cheese the Pidgeot xD). Yeah, I don't really have any rules of thumb for them. I just go with what feels right at the moment. ^-^