I've found that the weakest part of many presentations is in the transition from one point or idea to the next.

Shaky transitions are a prime cause of audience disinterest, and poor flow can quickly remove the auditor from the energy and flow the speaker has established, which makes such a time perfect for checking twitter and FB and otherwise disengaging.

The boundaries between ideas should feel natural and smooth, and I think for many it's difficult to arrive at a natural transition without getting very intentional about it, i.e. writing it out.

So I say write it down, edit, speak chunks out loud, and edit more.

In this way, the presenter can craft the most important sections with style while becoming familiar enough with the material that the delivery feels quite natural.