A Key snapshot when combined with a macro as Hosko describes is definately the most flexible.  If you program one of your transition buttons to be a FM Transition (you can choose which pair of FM channels you want to use) and save the condition as an M/E snapshot, the clip will be automatically recalled as part of the M/E snapshot - no macro or Shotbox required...in addition, you can program a specific transition (cut, mix, DME, or any wipe) for the background that can match the clip, and do this on a snapshot-by-snapshot basis with each M/E snapshot calling up a different clip.  Using the FM transition menu, you can set the starting and ending points of the transition, so each FM clip can have a different undercut point or can track the FM clip right to the frame.  Best of all, you can run the whole thing by just pressing the M/E's AUTO TRANS button - or have your macro press it for you.  This method works well for effects that you don't have to be flexible on - meaning that unless you use the ignore feature of KEY HOLD, it's going to recall all of your keyers & key states.  If you're doing sports or a type of show that you never know what other keys are involved on your M/E, use Hosko's method.  If you're doing a pattern show where specific things happen at a specific time, then the M/E snapshot works well.