Username generators can help, I've used this one a few times:
http://www.spinxo.com/And as for the "key components", I guess there's no wrong way to make a username, but I'd say to avoid numbers and/or things like "xXusernameXx".
Maybe try to stick to a few words. Some people have a word and an adjective before it, example: "LazySandwich". (I have no idea if that happens to be anyone's username, I just put together the first two things to pop into my head)
I guess you could just try and throw random words together and see what works.
