Proper use of English is one element to improve (this blog being a good example of what not to do).

In reviewing many powerpoint presentations, one simple mistake shows up a lot.
Inconsistent headers and formatting.

Here's how to fix it. Once you're finished with your presentation, take your entire powerpoint presentation, and flip through the slides quickly. Not 1 slide per second, but try it at 12 frames per second (Film movies go at 24 frames per second (fps) and TV does 32 scans per second). The reason for this is to catch any glitches, jumps, changes in font size, position, etc. The human eye will catch these changes, once you catch them. FIX THEM.

People will tend to see the differences as "movement". This is the LAST thing you want, since the movement will distract them as you're doing a presentation. They'll spend time wondering what changed, instead of listening to the information.