Most people would recommend more practice.. but I thikn it's these things that you will need to develop, how to do it depends on how you learn:
1. Technical - algebraic manipulations, differentiation, integration etc, you'll need to be reasonably quick and not make mistakes with these. Practice is proabbly hte best for this
2. Understanding - Go through the theorems and proofs and make sure you understand each stepo ftheir reasoning. Read the textbook sceptically and try to challenge each statement they make, the expected result is that you probably cannot find a reason to refute the books reasoning, but the process will gain you insight into the lines of thought.
3. "high level" thinking - Having a rough idea of of the result of calculations without actually doing them/ Making good guesses at what needs to be done. This is partly develpoed by experience/practice and partly by your mental arithmetic capacity, so when you study you might wantput away your calculator and proceed as much as you can before writing things down.