Here's how:
Use each finger as 0 or 1, and count in binary!
Naturally, a finger UP means 1, a finger DOWN means 0.
Here's an example:
512 + 256 + 32 + 16 + 2 + 1 = 819
Notice: the highest you can count is
512 + 256 + 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 1023
(but what if you use your toes ?)