Talk in a public place. No matter how well you know the person you are breaking up with, there is no way of telling how they will respond to the news. Take safety precautions to protect yourself in case the person you are letting down becomes angry, irrational or violent. Meet with them in a public place, like a restaurant, where you can talk seriously but are still among other people. Make sure someone you trust knows who you are meeting and where you will be meeting them in case you need to contact them for help.
Explain yourself clearly. The best way to make the person understand why you are breaking up with them is to explain your reasons clearly and fully. Be open and honest about your feelings. It's more respectful and honorable to explain why you feel things will not work out between you and the other person.
Let them talk. The conversation you have with the person you are trying to let down easy should not be a one-sided talk. Give the other person the opportunity to ask you questions about why you feel the way that you do and let them express themselves as well. Do not let yourself be disrespected or verbally attacked, but allow the person you are breaking up with to tell you how they feel about the break-up. This will let them know that you care enough about their feelings to listen to what they have to say.
Think it through. Before you decide to let someone down easy, make sure that breaking up with him or her is really what you want. Ask yourself why you don't think the relationship will work. You may discover after some consideration that you simply want to change some things about your relationship instead of ending it completely. If you are still sure that it's the right thing to do, tell them that when you break up with them. Telling them that you have taken the time to think about all aspects of the relationship and have put a lot of thought into your decision will show them that you respect them that much and will make letting them down easy a little easier.
Make it a clean break. One of the worst ways to let someone down easy is to break up with the person and then agree to see them again if they call later. Doing this will not make the break-up easier on them. It only gives them false hope that they can will you back or that the break-up isn't permanent. If you're sure that breaking up with them is the right thing to do, then do it and be done with it. You will both get over the break-up faster in the long run.