I bought a worm farm kit for the kids, and it recommended jumping around on the ground, hitting the ground, or possibly playing music over a speaker on the ground, as it simulates the sound of rainfall - to which the worms (supposedly) should be attracted. It did say that you may then need to dig down a little and that you're more likely to find them at night.
I can't swear to the effectiveness of this, as I haven't tried it yet, but may be worth a try?