heavy weights don't work for me, I need to drop the weight a little but really concentrate on good form... I go really extreme slow and try to flex the muscle i work out.
It's easy for the Triceps for me (for some reason I can really flex them good)(
Biceps is 50/50 I have some good workouts were I walk out and say, yeah I hit them right, and than some others were I'm utter disgusted with myself.
Shoulders, since I dropped the weight a couple of years back and don't give a shit if I work with small weight I see a lot of improovments, I say the Shoulders are the most wrongly trained muscle group ever.
The Back for me is the hardest to train right, I never know if I hit it right after the gym, I only know 2 days later.