Honestly, there is no easy way to say goodbye. Endings, good or bad, are inevitably hard. Let's face it, even the worst marriages have a hard time to letting go. It is even tougher, or messier, when one party tries to break free from the relationship while the other party is still enjoying. In very rare cases would the other party say with calm, "Oh, I understand that ... no problem."
My first advice to you is - don't put it off if you have made up your mind. The longer you make things drag on, the worse it will get. You are not doing any favor to anyone by waiting another week or month. The pain won't get less. If you do not see any future in a relationship, don't keep it hanging on.
Next, you need to be direct, as people appreciate directness. The best approach is to tell the other party that your relationship "isn't working" and then explain why. A well thought out explanation, including plain facts, will really serve the communication really well. So, don't beat around the bush.
If you want more advice on this subject, check out this site which is a great resource to help you with details.
