The more experienced you get with riding powder the more you completely avoid anything that gets flat (but let's be honest, we all get greedy sometimes).
I once ended up in a flat field in about 3 feet of wet snow, had to hike nearly 1/4 mile out to the cattrack. Suffice to say it took me nearly a 1/2 hour and I was fucking hotter than hell
Snowolf covered the jist of it, one method I have done though that worked well when I had to travel distance and dig someone out of the snow quickly was the commando crawl. By getting on all fours you distribute your weight better and sink less... pretty ridiculous but it worked.
PowderHound and TreeNinja