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.
Got stuck in powder once. Not pleasant. Almost lost my wallet because I forgot to zip my pockets so when you ride don't forget to zip/velcro everything!!
Last season was my first chance to ride real powder and on my first day accidently left my jacket pockets open. At about mid-day I noticed it felt like I was carrying 10 pounds of change in each pocket, reached in and noticed my pockets were STUFFED with packed snow.
Ok here is the best advice I have when your stuck look all around you if all you can see is pow then tell your self that your in Heven because where I'm from (Midwest) we don't know what that is so just count your blessing that you able to get stuck in pow and rember it's better to be stuck in pow than be in a place with no pow !!!!!!