Making a foot pedal
There are a bunch of foot pedals designed for transcription, such as the VEC Infinity: http://www.amazon.com/dp/B003L8081E/
They do a fine job, but are generally large and use USB HID.
For quick and dirty it's nice to have a single pedal that just generates keyboard commands.
You can select whichever keystroke you like, but PAGEDOWN, SPACE, PLAY/PAUSE or FXX might be a good choice.
Get yourself a trashed USB keyboard or if you must, spend $3 on a new one.
Take out the little circuit board, then find the correct two contacts that you need.
It's all trial and error but you can't blow anything up shorting any two of the contacts.
Buy a "sustain pedal", like the M-Audio one here: http://www.amazon.com/dp/B00063678A/
Remove the original cord and feed your USB cable through the bushing.
Solder wires from the two contacts that you found to the switch inside the pedal.
Hot glue or bolt the little circuit board down.