My experience is the opposite. I frequently edit pages nominated for deletion explaining their notability, expanding them, cleaning them up and provided sources.
Saving such articles on "Articles for Deletion" and noting you have made done so is by far the most effective method of ensuring the retention of an article. If a majority of people have voted to delete it based on the original substandard article, I leave messages on their talk page advising them that the article has been changed and asking them politely to have a fresh look at the article.
I've tried that too, but it didn't work for me. However, I forgot the part about leaving messages on the delete-voters talk pages. Also it has happened once that a major edit I did on a VFD:ed page was reverted because with my edit the vfd-nomination wouldn't be valid anymore. :) Instead I was supposed to wait until the page got deleted and then insert my new text.
In my experience I have found it hard to change someones vote. You either use the confrontational method "People who vote delete on this are morons, it obviously is notable. Check the 999000 google hits [www.google.com]" or the more consolidating "Maybe this article should be kept? It gets 999000 google hits." But people generally have a tendancy to not want to be proven wrong... Your experience most definitely has been different.
You also have very little time (4 or 5 days now?) to save the article. Many times I have skimmed through VFD:ed articles and thought that I definitely should "save" the article. But I can't with such a short time, and not with the threat of my effort going to waste.
-- mvh Björn