2D mode with heading up for me.
This is what I did when I had a GPS. It took a while to get used to since I like paper maps and the "north up" option seemed the right way to go. The "heading up" option requires a little less thought when translating what your seeing on the GPS to whats coming up. I tend to miss fewer trail crossings that way. The only problem is when you stop, the arrow reorients itself northward causing some confusion if you're lost and trying to figure out where to go next.