1. He can handle it. At least, he should be able to. Any man who wants to stand even a modest chance of success in the dating world needs to learn how to handle rejection, so don't shy from the task. Your concern for his feelings should never stop you from being up-front with him.
2. He wants to know the truth. He wants to be with you, but only if you reciprocate that feeling. You wouldn't want to be with a guy who was apathetic about you. Likewise, he'd rather know your true feelings than have your insincere acceptance.
3. He doesn't want his time wasted. If you aren't interested, he wants to start looking elsewhere as soon as possible (just as you should).
4. His primary emotion will be anger if you've only just met him or haven't been dating very long. Men take longer than women to get emotionally involved, so in the early stages of a relationship, his pride will be hurt far more than his heart or soul. While women will usually respond to this kind of rejection with sadness, men much more likely to be offended and angered, regardless of how well they hide it. So say what you have to quickly, and get the fuck out of there to let him cool off.
5. There is no way to soften the blow. This is what women (and probably men, too) forget the most when they reject a guy. You're telling him he isn't good enough for you, and that salient point will break through any bullshit you try to pad it with, so don't try. Additions like "I really like you, but..." are unnecessary and only make you sound insincere. The best way to deal with the inevitable is to plow through it as quickly as possible.
6. Most guys will persist at least a little. Don't be surprised or act indignant if he tries to convince you to go out with him, or to give you his number, or to stay with him. Men are rational by nature, and this kind of reasoning works on us, so we assume it will work on you too. Humor his reasoning by politely repeating your reasons (if it is a break up) or simply your refusal (if he is asking for a date or your number).
With that background, you should be able to appreciate the general strategy for rejecting a man, which is to be clear, direct and brief. That is to say, make sure he knows you are rejecting him, suppress the urge to make excuses for why you don't want him, and reject him quickly.
To state the same things in negative terms:
- Don't be vague in an attempt to soften the blow. Make sure he knows that you are saying "no" or breaking up with him.
- Don't make excuses about why you can't give him your number, or go out with him, or date him anymore.
- Don't try to soften the rejection by adding all kinds of caveats or compliments or by "talking it through." If you are ending a relationship there is a good chance he will want to talk about it, probably in attempt to reason with you (see #6 above), but I suggest you avoid this as much as possible. You can always talk to him later after he's had time to think about it.