S56 controller described a loss of separation event while providing OJT; the reporter acknowledging that he/she forgot over flight traffic during a technique discussion with the developmental.

Narrative

A Mooney was an overflight handed off from an adjacent sector; the aircraft was established on V257 flying south. A CRJ was inbound from the west and was cleared via the WAATS3 arrival to Runway 34L by the Developmental. I made a mental note of the potential situation as we had other more important issues to deal with abeam the airport as far as sequencing; spacing; and having to issue timely speed control and communication changes to the Final Controller. During the training scenarios we were discussing; I forgot about the overflight and when my scan went back towards the CRJ it was imminent that immediate action was necessary to avert the potential for a mid-air. A descent clearance to 140; traffic alert advisory and a 20 degree turn to the right was issued to the CRJ and a climb clearance to 160 was issued to the Mooney. A loss of separation occurred; 2.4 nm and 800 feet altitude. Recommendation; I should have told the trainee to do what I routinely do when I need to remember crossing traffic potentials; paper stop the higher aircraft by putting a higher altitude of the crossing traffic in the hanging field of the inbound aircraft.