If you don’t feel like paying full price for a foot trigger for your DSLR? You can make your own with a few parts if you’re comfortable with a little bit of soldering. Nothing major, but of course for someone like me, even opening a can of tuna is major. So I won’t be attempting this, but it’s cheap and easy enough that you could start after lunch and be done before Miller time.
Basically all you’re doing is switching the output on a simple on/off (not expression) foot pedal, the kind you’d use for a keyboard to toggle sustain. You trade the 1/8″ end of a 1/4″-1/8″ audio cable for a 3/32″ male, and boom, you’re done.
The full instructions are here; all the parts should be available at your local Radio Shack and/or music supply store.