Do your research. There are many books of monologues out there, but using monologue books for source material has its downside. For one thing, many of these books contain "original" monologues, meaning written specifically for the book, not having been extracted from a particular play. At most auditions you'll need to read something from an actual play. Also, monologues printed in monologue collections are often overdone and overused by actors. It's important to find fresh material, so read as many plays as possible. Sometimes a monologue from a serious play is just as funny as one from a comedy, so look in all sorts for places for new material.
Find something that makes you laugh. This might seem obvious, but if you aren't laughing at the monologue, chances are your audience won't laugh at it either. Make sure you understand the jokes, or what about the character is funny, in order to make those humorous qualities come across.
Do not use stand-up routines. Even though a stand-up comic does do a "monologue" of sorts when he or she performs stand-up comedy, that kind of performance is not a humorous monologue. Comedians are a great resource for learning timing and technique, but they usually write their own material and perform for a very different audience than actors do in the theater. Theatrical monologues should be from stage plays.