In wooded areas where the GPS goes nuts, I often look for a nearby clear area where I can get a good read of the sky.
I start there and begin slowly walking in the direction indicated, paying attention to the distance and direction.
Once the GPS begins to go bonkers under the tree cover, I put it away, do a little math (my paces are about 3 feet each) and try to pace off the remaining distance from the last clear read in the direction indicated.
It's not perfect, but it's usually more accurate than watching the compass and distance-figure bounce wildly under the tree cover.