another good way is to dig chunks of grass/dirt blocks. then it's easy to stack, then with loose clay dirt or something, mold the shit out of 'er. make it concave too. it also helps if you build when the dirt is moist, or the first sunny day after a good rainstorm or something. it's also easier to make gaps then tables. usually, when you make some dirtjumps, you want there to be a small jump at first to get speed, and make the jumps bigger and bigger.
Dig in the spring when the ground is wet or during a rain shower. Tamp it good and over a few days. If you build a jump and want to ride it the same day it will suck, good jumps take a LOT of work.