Tell someone "Suit yourself" when:

• you've tried to offer them something but they won't accept it
• you've tried to convince them of something, but they don't agree

"Suit yourself" means "I think you're making a bad choice, but do whatever you want." Here's an example:

A: Why don't you let me carry some of those bags for you?

B: No, I've got it.

A: OK, suit yourself.

"Suit yourself" sounds a little annoyed, but not too rude.