Whee!
Wait until it rains, or find a grassy park or sandy bit of land or something. Just ride and lock the pedals when they get to 3 and 9 o'clock (push down on your trailing foot, pull up on your leading). You'll skid a bit, and then you'll know what it feels like, from which point you can mess around. It works differently for different people, but the longest skids usually do come from a position similar to the one pictured above.
Also, if you change your gearing (bigger cog), you may find it easier to lock the wheels.