Impossible to do.....

[Rockport24]

If I can't pull it free with a decent amount of force (not enough to throw me out of the yak, I just cut it)
Before that though, I loosen the drag and paddle to the other side of the snag (I mostly get snagged on lobster pots and can usually just pull up the line and free the snag as long as no crazy lobstermen are around that think I am screwing with thier pots)

[FishermanTim]

I agree. I would try to shake it free BEFORE resorting to breaking/cutting the line. Hey, if I can avoid having to retie a line, I'm all for it.
One "caveat" regarding braid and snags.
If you are using ANY kind of braid, and you get snagged, whether you break off or are able to free you lure, ALWAYS check the line near the lure for signs of stress and wear. I have lost a few lures, fresh and salt, because I failed to re-examine my line after freeing a snag. Within a few casts after the snag my lure would go "Bye-Bye" and I would be forced to retie.