Avoid the temptation to quickly perform a movement and instead take your time. Learn to control the speed of each repetition. Think about squeezing the muscle as you lift. A good rule of thumb: perform the concentric (contraction) and eccentric (stretching) portions of the lift in two and four seconds, respectively. Slow down and practice awareness throughout each repetition.
BOTTOM LINE: Ultimately, what matters is not how much weight you can hoist (check your ego out the door) but how you can lift the weight with good form and feel the muscle working. Your purpose in the gym, whether it's to get stronger, leaner, more muscular, etc., is to lift safely while achieving your fitness goal. Train your muscles, not your joints!
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."