How can I pitch my startup idea engagingly to keep people interested?

Guy Kawasaki had this rule for Presentations - 10-20-30 and I will address that in reverse (applicable for a Powerpoint pitch):

• 30 is the minimum font size you will use in your presentation (makes you choose your words carefully, and besides people cannot read anything smaller)
• 20 minutes to sell the idea (If you cannot sell it in 20 minutes, the idea is not worth investing into)
• 10 slides is all you have (If you cannot spell your idea as a sales pitch in 10 slides, then chances are it will not get much attention)

If you think in the 10-20-30 rule, it will force you to be lean and to the point, something which people like and it will not put them to sleep.