How to Reject:
If someone shows his interest in you, and you do not feel the same, the best thing to do is simply to let the person know that you are flattered, but not interested. Expect him to be mature, and go on. If he isn't mature, that's his problem. Drama is always counterproductive. Always.
How to Take Rejection:
If someone you are interested in rejects you, move on. Yes, it hurts, but life is full of pain, and surviving is all about learning to accept the pain and moving on. I won't say something trite like, "There are plenty of fish in the sea." That actually may not be true. But there is more to life than sex, love and romantic relationships. And I suggest pursuing those other things.