My trainer taught me this secret. I do three sets per machine/strength training exercise. The first set I do 12 reps at the lower weight, 10 reps with the middle weight and 8 reps with the higher weight.
For example, if I'm doing the military/shoulder press I lift 25 pounds for 12 reps, 27.5 pounds for 10 reps and 30 pounds for 8 reps. It keeps me moving forward and those last few reps I KNOW I'm accomplishing what I'm setting out to do. I love this method!