Don't blindside someone. One of my exes did this with me. I had no idea we were having problems until he broke up with me. Let them know that there are problems with the relationship before breaking it off, and tell them you're thinking about it. That way perhaps you can both come to some mutual decision.
This worked really well for myself and my former boyfriend--he let me know he wasn't happy and didn't think he wanted to be in a relationship versus just breaking up with me out of nowhere. So we came to a mutual decision to end it. It felt a lot better than any other break-up I've ever had, definitely less hurtful and more productive.
The other thing--be honest. And the third thing--don't break up with someone two days before Valentine's Day or any other significant romantic holiday. One of my exes did that...I could've scalped him.
