Training tip - you should struggle at 10

My new personal trainer guy -- he's only mine for one more session, because I can't afford him full-time -- told me he prefers to train his clients to strength train using three sets of repetitions. Someone else once told me to do one set with heavy weight and to strive for momentary muscle failure. Which way do I go?

Perhaps there's value in each approach. I don't know. What I do know is what fitness trainer Jim Karas, author of The Cardio-Free Diet, tells Family Circle magazine -- that it's important to use the right amount of resistance when strength training to build more muscle and burn more calories. "By the tenth repetition, you should be struggling to lift the weight," says this exercise guru.

What are your rules for lifting?