good description of frame blending on Bob Donlon's Adobe blog.
On my very first day of doing digital effects, this was one of the first things I was taught.
Absolutely essential. He demos it in Photoshop, but you can do it in any compositing package.
http://blogs.adobe.com/bobddv/
if you're clicking on this after today, look for the post he made on April 09, 2007