I disagree with one point: don't start with writing out the talk. Otherwise the talk will feel unnatural, stilted... Instead, practice speaking your way through it - and write down nice turns of phrase that you come up with while talking. This will resolve most of the flow issues you'd get with starting with writing, most of the awkward sentences, keep the whole thing flowing naturally, and generally result in a better talk.
Preparing a talk by writing it out may work to some extent, but it's as unnatural as preparing an essay by speaking it out. The two are very different formats, and you should approach them differently.
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.
One thing I find useful for transitions is to set up some unrelated points in advance.
So instead of doing :
Thing a, thing b, thing c
I do:
We had to do a few things, then tie them together, First thing a, so we also had to do thing b. finally to get going we had to thing c. They all work together like result.
Another transition method that works for me is to just announce the seeming non-sequitor. "Now I need to talk for a minute about X. I know it seems unrelated, but it turns out to be vital..."
I find that I don't need to write down nice turns of phrase - if they're really nice, I naturally remember them, and can't help but hit them when I get to the subject.
Preparing a talk by writing it out may work to some extent, but it's as unnatural as preparing an essay by speaking it out. The two are very different formats, and you should approach them differently.