I work for Chegg and to improve your score on sentence completion, practicing with flashcards to learn vocabulary may help, as well as reading articles on the Wall Street Journal, Forbes, etc. that would have advanced vocab. Look up any words that you don't know and make flashcards for them. Chegg's GRE prep course also includes a mobile app with over 500 flashcards so you can study them anytime, anywhere. We also offer personalized diagnostic reports to easily identify your strengths and weaknesses on the verbal section, so you don't waste time on concepts you already know. The course also includes a quiz bank of over 2,800 questions where you can design quizzes to focus on your most challenging areas based on difficulty, topic, or length. If you're interested in book prep for the quant section, we also have an ebook with a bonus practice test and over 300 additional practice questions.