Slang: do you love it? Hate it? Find you can't avoid it?
Using lots of slang can date your work, or at least your characters. That can be a slippery proposition in some genres. You can make it work for you, though. My title example, "cat's pajamas," comes from what era?
If you guessed the jazz age, you're right. Using old-timey slang in dialogue can be a fun way to suggest, for example, that Great-Granddad wasn't always a funny-smelling Jeopardy addict with dentures. Perhaps back in the day he was the popular guy every girl swooned over.
Slang can also suggest your character's ethnic background and class. How heavily you rely on it will, of course, depend on genre.
One of my favorite sites for slanging up scenes that involve British characters is the Peevish Dictionary of Slang and Colloquialisms Used in the United Kingdom (I also double-check with real people who live in Britain, just to be sure I'm on target). It's an especially useful site because it's searchable. Pop in a term like "crazy" and get useful results like "barmy," "nutter," "potty" and "totally hatstand." Even if you don't have British characters, it's an awfully entertaining site.
Do you use slang in your work? How? Any favorite sites you frequent to research it?