i was at my hill today, where they too were trying to make a jump out of shitty ass powder snow (for packing. i love da powwwww). anyways our park manager had a propane tank with a torch hooked up to it that looked like this. he said you could find one for 15 dollars at home depot and they're really easy to use.
it melts the snow, water runs down the snow below, that water freezes, voila: harder jump. although the rest of the suggestions in this thread are accurate and work as well if you don't have the resources for this.