There are a few things you can do:
1. Use flashcards to study vocab if English is not your first language. Not just random vacab but words you run into while preparing, in texts, questions, etc. While there is no link (supposedly) between GMAT Score and vocab knowledge, it helps to expand your vocab if you are running into unfamiliar words
2. You can use the flashcards for specific concepts, formulas, questions, etc
3. Idioms - this is helpful
4. Lists for grammar (e.g. words that are always followed by infinitive or subjective mood words, etc)
5. Perhaps the most valuable and helpful to me (but time consuming) was creating my own notes. You could create your own flashcards. A number of members have created those.
https://gmatclub.com/forum/gmat-flashcards-108651.htmlThanks
BB.