I read somewhere that you make an X with your poles, hold in the middle and push yourself out after some digging.
But i wouldn't know cause i'm from the east coast where the snow doesn't get high enough for me to ever worry about that unless i'm standing by a street when a snow plow drives by.
Depends on the slope and situation... I'd much rather be stuck on a steep slope than in a mellow section.
If you're in maybe waist-deep or less and have big skis then just make a big pit/snow angel... any more than that and shit I dunno, last time it happened to me I ended up using 1 ski as a sort of raft and wallowed through it until I found a bush in the snow, climbed on top of that and got going again...
Put the poles flat on the snow, and use the to push your way up (that's if you're not really buried, just stuck laying on your back and unable to get up. If you're buried, I guess you'll have to dig a bit then use the pole thing
If your skis arent to buried what i do is get them perpendicular to the slope, push them down into the snow a little more and use my poles to push my way up into a extreme backseat position. Then just get yourself up with your skis under you.