Before you are ready to attempt a parallel bars knee raise, you’ll need to have the strength to hold on a parallel bars for at least 20 consecutive seconds. If you can’t do that yet, practice holding up for as long as you can. It shouldn’t take too long to build the endurance needed to begin.
Pull your shoulders back and down don't let your body to fall down in a way that your shoulders will be close to your ears. This helps protect your shoulders from injuries.
It is very important not to use momentum on this exercise, Make sure to move your legs slowly, swinging your legs and torso will make your workout much less effective.
Make sure the movement occurs in your hips and not in your lower back.
Using a parallel bars with a back pad like on a Captain's Chair will help you to maintain a proper position and avoid swinging your back to do the lifting.