I had to fix a flat on the road just the other day and the pump I used was a Zefal HPX3. Attach the pump to the valve, grip the pump near the head firmly in the left fist while bracing the left fist against the rim and the rim against my thighs and pump away. Force is taken up by the fist, rim, and thighs. I have one of those little Topeak pumps on another bike and they are much easier to use and less likely to snafu the valve, but the old frame pumps, like the Zefal, do the trick too.

Personally, I have more trouble with Shrader valves than with prestas but most people seem to prefer the opposite.