It really depends on your personal situation and how much. If you're in the U.S., you'll have to check to see the min/max for suits in small claims court (this is generally anywhere from $3,000-$15,000). One thing that works, though, is merely having your lawyer draft a letter to the offending client. Your less intelligent clients will jump when they see any threat of legal action (and for you, it's only a few hundred bucks to get a lawyer to draft a letter – maybe even free depending on your relationship). There's certainly leg work involved, but when it's your livelihood, you have to do what you can.

Stipulate in your contract that the losing side has to pay the legal fees for the other party. That way getting paid doesn't cost you more.

Use a collection agency, if you can.