Know the reasons why you're breaking up, because if your partner isn't expecting this piece of bad news from you, you'll end up doing a lot of explaining. So it's good to go well-prepared. It also gives you an opportunity to reflect on your thoughts and see if a break up is what you really, really want.
Do it in person.
Do not break up over email, SMS, or telephone, unless the situation demands it. Meeting someone for the last time before the relationship ends can offer a good way of closure, which is psychologically important to move on.
Do it in a place that's comfortable for your partner.
A crowded restaurant with a lot of strangers around might be intimidating. Pick a place that makes them feel safe enough to express their emotions.
Don't lie to your partner when you're about to end the relationship. Tell them exactly what you felt was wrong in the relationship. This might be harsh on them but equally important for them to learn. At least they'd know where it all went wrong.
Don't be cruel.
Break ups are terrible any way. So don't go out of your way to make it more hurtful. Remember that you were once in love with this person. Being honest doesn't mean you have to be vindictive and cruel. Try to deliver your truthful words in a nice package.
Don't be ambiguous.
Don't leave your partner in doubt. If you don't mean it, don't tell them something like, 'it's not working out now, but it might work out another time.' That would wrongly make them hopeful.
Be prepared for the worst reaction.
Being dumped is perhaps one of the worst things that can happen to anyone. So don't be surprised if your partner starts shouting or cries or argues or just storms out on you. Be calm.