What should I do if I can't complete the reps in my weight-lifting workout?

If you miss your target workout repetitions by more than one repetition (rep) on the first exercise of a given workout, go home, take the next day off, then repeat the workout.

Let's say you're scheduled for a workout on a Monday.

Workout:
Pull-down: 8 reps × 80 lbs p 8 reps × 110 lbs
Machine shoulder press: 8 reps × 30 lbs p 5 reps × 60 lbs.

The first exercise is close-grip pull-downs, and your target number of repetitions is a minimum of seven. If you complete six good repetitions or more, complete the entire workout. If you don't complete six repetitions for pull-downs, do NOT proceed to the shoulder press.

Instead, pick up your gym bag and go home. Rest Tuesday, ensure proper nutrient intake by eating a ton, and come in Wednesday prepared to crush both exercises and proceed as planned.

If you fail before the requisite number of reps, do not -- as many people do -- decrease the weight and do another set (called a "drop-down" or "break-down" set). Do nothing but leave. If you haven't recovered, you haven't recovered. Continuing can easily stagnate you for two weeks or more.

Cutting a workout short takes tremendous self-control and runs counter to gym culture. Be smart and opt for a 48-hour reboot instead of a two-week or three week reboot.

Last but not least, if you abandon a workout because you miss a set, add another recovery day between all workouts moving forward. In effect, you're just accelerating the planned decrease in frequency. There is very little downside to doing this. Twenty-four hours of additional time cannot hurt you, but underrecovering will damage the entire process.