Every time I did these I got 98% or higher. I only got worse grades when I didn't have time to do all these things because of how many classes I was taking simultaneously.
Start doing them as soon as you get them. Never spend too long on any one assignment or question. If one question is taking too long, ask for help as soon as you can. Practice getting and giving help, these are crucial skills. If necessary, cut your losses (but always turn in your best current attempt for every problem, never miss anything).