Watch cartoons! This is one of my favorite tips! :) I love cartoons. The thing to note here is the major differences between cartoons and animated movies. The big block-buster films from Disney/Pixar or Dream Works typically lack the more zany and quirky voices that are more common in your Saturday morning-type cartoons like “Sponge Bob Square Pants”.
Sometimes even those cartoons will have characters with “real sounding” voices rather than cooky ones, and some have a mixture. Look at “The Last Air Bender” for instance.
Anime is a whole other genre of animation with a mixture of real and “cartoony” voices.
Watch all kinds of animation and see which ones you like the best.