For example, lets say you are currently doing 3 sets of 8-12 reps on bench press with 200lbs but you can't get passed it. Modify your workout as follows:
Week1: 180lbs, 3 sets 8-12 reps.
Week2: 185lbs, 3 sets 8-12 reps.
Week3: 190lbs, 3 sets 8-12 reps.
Week4: 195lbs, 3 sets 8-12 reps.
Week5: 200lbs, 3 sets 8-12 reps.
Week6: 205lbs, 3 sets 8-12 reps.
Another option that I would consider if I were you is to adjust your weight upward and lift for lower reps. For example if you are currently lifting 200lbs, 3 sets 8-12 reps, perhaps try 215lbs for 6 reps. All of that being said, if you aren't getting enough protein and aren't getting enough rest none of this will help. If you are feeling lethargic going into your workouts or are feeling unusually "heavy" and tired throughout the day you are going to need to make a change.