My number one recommendation is to lift heavier weights.
Period. Full Stop.
"ok Jill, tell me more!'
When you participate in a class and an instructor says, 'we are going to do this for one minute,' you probably select a weight that you will be able to do for the entire minute. Try a different approach and choose a weight that is just a little bit heavier and try for the minute. If you reach failure at 45-50 seconds, keep using this new weight. The more you challenge your muscles, the stronger they will build up on the other side.
Lift HEAVIER weights!
Start with bigger muscle groups earlier in your workout. So start with hamstrings, glutes, quadriceps, back, and chest and heavier on those and then move onto your smaller muscle groups which consist of shoulders, triceps, biceps and calves.
Create variety in your workout. If you always squat, only the squat primse movers will get stronger. If you want stronger legs try squats, deadlifts, lunges, and lateral movements.
K.I.S.S. Keep it Simple Silly :) Pick a weight that challenges you. Goal is 3 sets of 8 to 15 reps on each set. Perform this routine at least twice a week.