Go to your professor's office hours. Seriously. Almost nobody does, and the ones who do get better grades and better recommendation letters. Professors WANT to help students who show up and care. It's the most underused resource in college.

Teach it to someone else, even an imaginary student. If you can explain a concept simply and out loud, you understand it. If you stumble, you've found the gap you need to study. They call it the Feynman technique and it really works.

Form a study group, but a small one — three or four people max. Explaining something to a classmate is the best way to find out whether you actually understand it. Just make sure it stays a study group and doesn't turn into a hangout.

Strunk and White's 'The Elements of Style' is a tiny book that will make you a better writer almost overnight. Every student should own a copy. It's cheap, it's short, and the advice in it never goes out of date.