A spacebar. It takes forever, fiddling around getting a map in and out of my pocket, and doesn't help with the whole 'wrong turns' business. I kept Deano waiting for hours last week after my unscheduled visit to Walltown Crags. He was so nice about it, but it was the difference between a soaking and a not-soaking. With my map easily visible I'd have checked my position earlier.
Ah, well, live and learn. A map holder made audaxing much easier, and it is sure to help on the hols as well.