1. Be honest.
2. Be direct, don't bullshit.
3. Face-to-face in a public place. That way if they make an ass out of themselves, its also public humiliation for them and helps you not be swayed by emotional outbursts.
4. Even if you might love the person, there's a reason you want to break up so keep that firmly in mind and don't give in.
5. If you're a douchebag and have been:
a. Cheating
b. Lying to get in their pants.
c. Emotionally mistreating them.
Then you are not entitled to any sympathy when breaking up and as stated before are a douchebag.
6. Don't give them any signs of hope, because if it was going to work out, it would have before this point.
7. DO NOT GO AND DATE SOMEONE DIRECTLY AFTER BREAKING UP. This means you are a douchebag, untrustworthy and non-committal. Take time to appraise your situation of being single, enjoy it for as long as it takes to make you feel comfortable being by yourself. Breaking up with someone is not license to jump to the next relationship to make you feel better. It means most likely you are co-dependant and have issues that will prevent a successful relationship.
8. Don't be an asshole no matter how bad the person might have treated you. If you want to walk away being the better person, then BE the better person.
9. Don't try taking things you gave the person as gifts back from them, that also makes you a douchebag.
10. If there's money involved, write it off as a bad investment unless you think you can settle it in court (and want to afford a lawyer) or have any way of proving culpability of the other party. Otherwise, don't expect it back, because you'll be tied to that issue possibly for years on end.
Basically if the relationship is over, end it and don't try to walk away being friends. It doesn't usually work out well. Be an adult, know when its over and accept it.