There's a reason "Death by PowerPoint" is a well known and roundly recognized problem. Despite their occasional dullness, Word, Excel, QuickBooks, and the rest, are rarely charged with bringing users to the brink of their own demise. Sure, bad spreadsheets are annoying. But bad presentations can be ridiculously painful.

As an entrepreneur, you still probably need to make presentations regularly in order to sell your ideas or train your staff, but like dangerous weapons or flammable liquids, presentations need to be handled with care.

Luckily, there is research available to guide you away from inflicting undue agony on your audience. Behavioral psychologist Susan Weinschenk is at work on a forthcoming book that explains the science of presenting and aims to help even novice presenters make their point without tormenting their audiences. The book, entitled 100 Things Every Presenter Needs to Know About People, is due out in May, but Weinschenk is now previewing some of the principles it contains on her blog.

Among her advice is the simple but powerful truth that duration matters. Even rock star presenters (consider some of TED's most riveting speakers) are only given 20 minutes to make their case. Apparently, there's a solid scientific basis for this duration, according to Weinschenk:

20-minute presentations are an ideal amount of time.  Maureen Murphy tested this idea in an experiment. She had adults attending a 60 minute presentation at work, and tested to see the difference in memory and reaction to the same talk given in one 60 minute long presentation, versus a presentation that had 20 minute segments with short breaks in between. What Dr. Murphy found was that the people enjoyed the 20-minute chunked presentations more, learned more information immediately after, and retained more information a month later.

So, for the love of your listeners, take this lesson to heart: 20 minutes is the presentation-duration sweet spot. This is true even if you have more than 20-minutes worth of material to get through, insists Weinschenk, who suggests the simple solution of thoughtfully planned breaks. "If you are presenting for more than one hour you probably have a break planned. Time the break so that it comes at one of these 20 minute time periods," she writes, and goes on to caution against the temptation to simply break your material in half:

Instead of taking one long break, take several short ones. For example, it is common for a half-day workshop to go from 9 to 11:30 or 9 to 12 with one 20-30 minute break at around 10:30. Instead of one 30 minute break, have one 15 minute break and then 3 other short 5 minute breaks.

What other golden rules of presenting do you follow?