Discuss n's answer to: Heartbreaker's Guilt

Look the person in the eye and say something flattering about them, something you honestly like about them.  Then add the "but" and continue with the same soft tone of voice and say something like:  "but you know...I simply do not feel that way about you.  You are a wonderful person, and I wish things could be different for you, but I have to be honest with you...."  and continue until you've said what you feel needs to be said.  Then after that, if he continues hanging around or calling etc, you will have to (perhaps..I hope not) use stronger language and a sharper tone.  It is a difficult situation, but you've helped him the best you can, and you must always be true to your own heart.  He would not want you to lie and pretend just in order to let him down gently.  It is best to be honest with him, but do so in a kind way.  Many people cannot do that, and you may also end up in a shouting match.  If that does happen, don't blame yourself, because you tried to do it gently and it just didn't work out that way.  Forget the episode and move on with life.  That's my advice, from my past experiences.