You can probably appreciate becuase it sounds like you have a lot of the voices down already, but as with almost anything, I've found that "practice, practice, practice" produces the best results. I spent most of my free time the last two years in college listening to Sesame Street records, watching vhs tapes and talking along with them, ebay was my best friend in the late 90's. If only I had started practicing my puppetry skills with all the puppets I bought while imitating the voices!
Other times, it just takes a while to get the voice. There are several characters that I couldn't figure out how to do, and then months or years later, out of the blue (perhaps spurred by a certain audition). . . "hey, that sounds like Gonzo!"