Contrary to popular belief, it is actually possible to end a relationship without hurting the other person's feelings in the process. The problem, however, is that the process is a long one and requires a level of personal integrity and absolute honesty most people are unable to muster.
Ideally your relationship is one where honesty and open communication is not a problem, and where a desire for the other person's happiness is the most important objective of the relationship for each party involved. So long as you are fortunate enough to have found such a relationship with someone, and consider yourself lucky if you have, you have everything you need in order to end the relationship gracefully while sparing the other person's feelings.
The first order of business will be to determine for yourself exactly why it is that you want to end the relationship. Is it because you feel stagnant? Are you afraid of commitment? Are you interested in exploring opportunities with other people that require you to be a little less attached? Whatever you reason, be sure you know what it is and be prepared to articulate it to your (soon to be) ex.
The next thing you need to do is to wait. That's right, you heard me. You need to wait. Never make a drastic decision like this without giving it serious thought - and that means give it some time to contemplate what it is you really need and whether or not a break-up is the best way to accomplish it.
In many cases it may not be, although it may take talking with your partner to realize that. So, after some time of contemplation if you still believe you might need to make this decision, it is time to approach your partner and talk about it. Do not wait to have this conversation for too long, as denial and resentment can end up causing far more problems in the long run.
Since you two have that open and honest relationship with each other, this part will actually be the easiest part of the break-up. You start by talking about how you are currently feeling. You explain what it is that is lacking and why it is that you need to make a change. What you do not do is make any unilateral decisions without discussing and arriving at the conclusion together that's how you got into this relationship, and leaving it should be no less respectful and cooperative. If you have the right type of relationship and you have initiated this process before making a final decision, you end up in a wonderful situation of being able to leave the door open for what u've actually need