The first time I made a credit button, I used thick foam double-sided tape to hold the coin door closed, and took the lock mechanism out. Then I found a button that fit the lock hole and used that.
The next time, I put the tiniest button RadioShack sells inside one of the coin return holes at the bottom of the door, so it was invisible from the outside, but you could just stick your finger in there and hit it.
Since then I've made the coin return buttons trip a switch, like Chris said.