I've been in a lot of relationships. Some short-term, some long-term and some simply way too long, one of the benefits of dating is not just learning the ins and outs of being in a relationship - but also how to get out of one gracefully. I'd like to tell you that there's a secret formula for calling it quits, like taking the square root of the number of relationships you've been in, divided by your age, minus the number of bridesmaids you will have at your future wedding - but it's not that simple. Trust me. If it was based anything on numbers, I would have written the book on this a long time ago.
What it does seem to be based on, however, is a willingness to learn about yourself from those past experiences. Acquiring the self-awareness that breeds a certain kind of wisdom and trust in yourself required to know when to let go. And through this strengthening of self, and the careful refinement of your standards that inevitably results, saying goodbye doesn't have to be the roller coaster it once was.
Let's assume you're the bearer of this bad news. Telling someone you don't like them anymore is one of the trickier aspects of dating - right up there with telling someone how much you do. Any kind of candid disclosure that involves the expression of feelings - or lack thereof - isn't exactly the type of thing that rolls easily off the tongue for some people. But in either case - if not more so in this one - it is always best to be honest.
But there's no need to be harsh.
For example, "Honey, I'm leaving you. Your feet smell like rotting flesh, and if I have to hear your rendition of 'Bad Romance' one more time, I'm going to gouge my eyes out with a spoon," may not be the most tactful way of going about this. Also not OK: "I don't love you anymore - but I love your dog/cat/fish/uncle/car/sister/mom!" But to each his (or her) own.
Effectively and compassionately communicating your disinterest in someone is not only necessary out of fairness for yourself and the person involved, it is a sign of emotional maturity.
As is gracefully receiving such news.
The easiest way I've found to deal with receiving love's harsh realities is simply to re-frame it in my head. This person isn't condemning you to a lifetime of lovesick hurt, anger, bitterness, jealousy and rage, he is freeing you to be with someone else who actually will love you. So when you're done feeling all of the above - which, by the way, is best expressed to your dog/cat/fish/uncle/car/sister/mom rather than the person himself - find it in yourself to forgive him. Heck, even thank him. (But again, I recommend probably also keeping this one to yourself, lest look like a passive aggressive maniac.) Then, move on.
Whether you're nearing the end of a casual encounter or something long-term, just remember it's not supposed to be easy. With a little re-framing, self-assessment and faith in the future, you'll see you're just one step closer to the one who's meant to be.