First, develop a technique for undoing the hang-up. My experience is if you know you're into a tree, let slack off immediately, unless you are already completely taught, in which case you hold the flie and line taught until you can have a look at how it is snagged. Often, at this point slowly allowing a relaxing of the tension may demonstrate that you are hopelessly wrapped, or perhaps only temporarily suspended.
If the latter, try using very gentle flicks of the rod tip that introduce and take up short 6 inch segments of slack. Sometime: "flick, flick, flick" and it's loose. As noted, do not yank on it, but just flick the line to draw and release a short segment of slack.
No go? Let another 10 feet of slack out and grasp the line above the rod tip and get the rest of the rod out of the way, straightened fully, with a little slack so you don't end up re-bending the rod. With line in hand, work your way slowly toward the snag. When close enough, use a nearby stick or your wading staff to draw the branch into your hand, where you can bend the bow to eye-level. Remove fly. If the branch is still too high and you cannot climb the tree, with your hand as close to the fly as possible, pull straight until the tippet snaps.
If you don't mind leaving a fly and 3 feet of leader hanging from the tree, then some of the other ideas for snapping the line may work from 30 feet away.
