first watch some of these to get a feel for what works and what doesnt, watch this video on how to present like steve jobs. Read Pitching Hacks Make sure you have a high concept pitch, elevator pitch and slide deck
Attend a local Toastmasters: a) you get to practice and get comfortable with public-speaking, which is much more important than practicing the pitch, and b) you speak with a variety of people, and if they "get" your pitch, then you can be assured people in your market or investors will also get it.
Oh and, don't practice on friends or family, unless one of them is an investor, and don't practice on throwaway investors. None of them are throwaway, because they talk to each other.
The best way to practice your pitch is to always be pitching. You dont just pitch your idea to investors, but your friends, family, partners and employees. You want to do this because the more you do it, the more refined and less nervous you will be.
You should practice your pitch on the least interesting investors first. If you have an IT company, try pitching for an investor who is into farming. This is better than bothering your friends and the guy is likely to come with some of the same points as your real investor, meaning that you will get somewhat relevant feedback without wasting the most interesting ones. Once you feel more comfortable, try pitching investors that are more and more intersting, meaning that you will get more and more relevant feedback.
I suggest you look on meetups.com for networking/startup groups in your area. The people attending these groups more likely than not will be interested in learning about your business, and will hopefully ask you good questions, as well as challenge you on the parts of your pitch that don't make sense.
Also look for SCORE sponsored events in your area. A lot of the people that attend the SCORE events are your local small business type of folks (e.g. carpet cleaner, maid service), which may not be the most technically savvy. This gives you the opportunity to practice your pitch on the "average consumer". I recently attended a local SCORE event that I think helped me with my pitch.
Also, I've found that it's best not to over-prepare. Don't try to memorize your pitch word for word, because it will sound robotic and rehearsed. You want to sound natural, and comfortable with your words. Just focus on remembering the important points. At least that's been my experience.