I learned a lot about pitching from this great article by Craig Mazin. I think his best piece of advice is to imagine that you're spinning the story to someone as part of a conversation and make it natural. Mind you, this is easy for me when I'm in a pub yakking on to my friends and really hard when faced with two gimlet-eyed Hollywood types in their Prada suits!
I think you're either naturally good at pitching or you're not. If you're like me and it takes a lot of work, it does help to know the script inside out and to be passionate about it. If you don't love the script, it will show in your pitch performance. The other thing to bear in mind is that high-concept ideas are easier to pitch. I love my family horror-comedy but it's a nightmare to describe in a few paragraphs. The initial premise is slightly complicated and it takes a lot of build-up to get to the good stuff people laugh at. If you have more than one script that you're happy with and one is high-concept, that's the movie to pitch.
There's really no easy way around pitching: read your script again and again, tell the story to anyone who'll listen and watch for the parts they respond to. Record yourself pitching and carry it til it's right. Remember, this could be your big break!