A pitch is the business equivalent of a Plenty of Fish profile. You want to present enough information to get a first date, preferably with someone who's not a complete nutjob. A short, clear overview is much better than a long, detailed breakdown. Make sure your last slide has contact information on it. Your pitch should invite people to ask questions.
You should learn to do an investment pitch and/or a marketing pitch all over again at least once every three months. Why? Although your elevator pitch may remain relatively stable once you've figured out your company vision, the people you pitch to will all be different, and your company itself will change pretty rapidly. You need to be on top of what your company needs, and you have to learn to read your audience so you understand how to present your needs and goals as complimentary to theirs. This will take a long time to learn and the best way to learn is to practice.
Pitch contests are a valuable way to get experience and feedback (and sometimes there are prizes). There are a lot of great resources online, too - look at decks of successful companies that are similar to yours. Also, look at who eventually hired/invested in/acquired those companies. Keep the culture of your company and the location of the audience in mind as you design your pitch. Something that works in a pitch for an MRI system may not work for a mobile app.
Here are some pitch resources that I've used:
A great analogy describing exactly what investors look for in a pitch:
Practicing your pitch will help you feel confident when talking about your company to complete strangers in airports and while waiting in line at the grocery store, which is incredibly important. You should be able to explain what you're doing to a stranger in under 30 seconds.
If you don't have startup events and pitch contests in your town, you could organize a pitch practice event and invite other local startups to attend. You can also record your pitch practice and put it online, and ask for feedback from bloggers who write about startups, other entrepreneurs, and the general community. Most people will be happy to give you feedback if you explain that you are from a small town. In fact, there are now organizations like (startups everywhere else) specifically designed to support you.