My favorite is to find the switch labeled "Zero Pos Replay Unit" (on the credit/replay unit) and short it at the switch solder tabs with a brightly colored alligator (or EZ Hook) based jumper. A jumper that is easily removable to return the machine to stock configuration. No switch mangling or soldered jumpers...

That being said, I rolled my two half moon credit unit machines out of the lineup to snap a picture... and one had a soldered jumper, the other a mangled switch. I could get you a nice pic of a reel based credit unit with alligator clips tho... sigh...