If you want to succeed at building mass and strength through weight training, you need to train with enough intensity. Just going through the motions and not trying to do little more weight now and than probably won;t get you anywhere. You need to use some high intensity weight training techniques every so often. One of the best high intensity techniques to use in a weight training routine is the forced reps technique.
Doing forced reps is pretty. Basically, on certain exercises where you use a spotter, you have them help you out with a couple of extra reps after you've done as many as possible by yourself. his is a great way to go beyond failure and get some higher intensity weight training in. You won't want to do more than1 or 2 forced reps, however, as doing more will probably tax your spotter more than you since they'll be doing more of the work by then and you could also end up overtraining or even injured. You want to take your intensity level far enough to get a response, but not so fat that you end up overdoing it.
You can use forced reps on many major weight training exercises. Probably the most popular exercises to use them on are the bench press and incline bench press. These are 2 exercises where it's relatively easy for a spotter to assist you in getting some forced reps by lifting up a bit with their hands on the bar.
Military and dumbbell presses are also easy to have a spotter help you get some forced reps. Have them spot you on the military press like they would on the bench press. For dumbbell presses have your spotter put their hands under your elbows to help you get some extra reps.
Pulldown exercises for the upper back are also easy for a spotter to help you by pushing down on the pulldown bar. You can even have a spotter help you get some more reps when you do pull-ups. Just have them put their hands on your waist and lift a bit to help you up. You may not like the way this looks in the gym, but it will help you hit your lats with a lot of intensity.
Many bicep and tricep exercises are also easy for somebody to help you get some forced reps. Have your spotter stand in front of you for bicep exercises and behind you for triceps exercises to help you squeeze out a couple of forced reps.
Of course there are some exercises where a spotter won't be able to really help you get extra reps. Major compound leg weight training exercises like squats and lower back exercises like deadlifts are just to difficult for a spotter to help you get extra reps on. If you want to get forced reps on leg and lower back exercises, you can have a spotter help you when you do them on certain weight training machines. Have them push up on the plate loading bars on a hack squat or leg press machine to help you get more reps. Leg extensions, leg curls, and some lower back machines can also be great for getting some forced reps on.