I am amazed at how many people I see at the gym who simply have it wrong. They are banging away on exercise equipment without the clue for how to use to correctly.
For example, doing leg raise on parallel bar type equipment. Slamming their legs up and down fast.
Really you should be raising knees to chest or doing an "L" slowly then dropping the legs back slowly afterwards.
This is just one example where you will minimize the benefits you get and make yourself frustrated.
Go for slow, smooth movements that maximize the muscle use and engage it fully.