i always pre exhaust on the bigger muscles that tend to incorporate other minor muscles...(back, chest, quads, shoulders).. for me i find that the best pre exhaust exercise for back is doing high repetitions on smith machine dead lifts..then mooving straight over to machine pull downs. for shoulders i think it depends on what yoru probelm is...if your triceps tend to get too invloved, the doing dumbell laterals as a pre exhaust method would work great; if your traps tend to get too invloved, the behind the neck smith machine press is a great pre exhaust method. for chest, dumbell flies and cable crossovers are great, for quads i always go with leg extensions.