To get $20 off $100+ Star Wars Stuff at ThinkGeek you first have to spend up to $100 on Star Wars stuff before shipping, which shouldn't be too hard. Next, you'll need to use the code USETHE4THLUKE (har har!) to trick the shopkeeper into thinking that these are not the dollars you are looking for.
The sale goes from May 2nd to May 6th so make sure you hurry up and go grab your Star Wars Products from ThinkGeek ASAP. I guess Mother's Day is around the corner so you might want to add a Wookie Bathrobe to the order for her...which you'll conveniently buy in the "wrong size" and borrow because she didn't like it anyway.