Paul Graham has another great essay.  This one's on Stuff -- and more particularly, how to acquire less of it.  Here's my favorite quote:

Another way to resist acquiring stuff is to think of the overall cost of owning it. The purchase price is just the beginning. You're going to have to think about that thing for years—perhaps for the rest of your life. Every thing you own takes energy away from you. Some give more than they take. Those are the only things worth having.

I can't think of a better thought to have before taking that next client.  When you take a client -- especially that client that your gut tells you not to take -- think about the overall cost of having that client.  Don't focus just on the money you'll make from them, but how you'll feel while working for them.  Will their file keep you up at night?  Will you dread their call?  Will you be able to give them your best work?   Search for clients whose personality matches yours and whose work challenges you to do your best.  They are the only clients worth having.