There was someone I really, truly hated. He was practically the opposite of what I thought a good person should be like. It's been awhile now, and I've let go in two stages.
First, I turned the hate into pity. I pitied him; he'd never know happiness or success in the manner I would, so long as he remained the same.
After that, it was easy. For me at least, the pity was easy to forget about; it was easy to forget about him.