Ask yourself: Does the idea fit in with what the company already makes or what the investor is currently interested in? If you had a new idea for a novelty athletic shoe, pitching it to Levi Strauss wouldn't be your best bet--they don't make shoes. However, Nike or Stride Rite might be interested. If they are, it's because they see your product fitting in with their core business and current product line.
If a company sees a good fit, you'll have its attention and an excellent shot at selling your idea. Along these lines, it might be a good idea to mock up your idea in some of the company's packaging. For instance, if you designed a new power tool, you'd go to the hardware store, see how the company's power tools were packaged, then create similar packaging and place your prototype in it. This attention to detail looks very good in a presentation and demonstrates that you've researched the market.
Presentations are key to communicating and selling someone your idea. They also tell people what kind of person you are. Because they're so important to the future of your idea, think like a Boy Scout: Be prepared.