I started studying in August to apply to master programs starting in 2019. Like many people, I picked up the Manhattan set. They are good books and I would highly recommend them, especially the quant ones. I went through all the OG books after learning the material from Manhattan and scored a 630 on my gmatprep. I knew I would be satisfied with a 650 so I went ahead and took the real thing. Instead of scoring 20 points more, I scored 10 less and ended up with a 620 (q45 v31). I found the score to be a little weird since the quant score I got was my highest ever while my verbal was the lowest ever. After the 620, I knew I needed some help outside of books but also did not want to spend a lot of money. However, I ended up getting the empowerGMAT course ($85 for the first month and $99 for the next), gmatprep exam pack 3&4 ($40 through a deal), and a Kaplan book for more tests ($25). Oh yeah and the $250 for the GMAT retake. Yikes.
I paced my self to finish the course in 6 weeks so it lines up with my retake date. Overall, the course helped me the most with test-taking strategy and how to set up the work in the verbal section. In quant the best tip I found was to get all the answerable questions, dump the really hard ones. The ones you dump actually have to be considered hard by the test. These, for me, were crazy inscribed geometry and crazy combination problems. I knew I that half of the times I would get them wrong and even if I got them correct, it would take me 3 mins or more. For verbal, I found knowing how to approach each question type, even types of questions within SC, RC, and CR, gave me a lot of confidence. Knowing each type of question is highly beneficial even for native speakers. I found the course to be much more useful than the books I used, not to say that the books are bad. I got a 690 (q47 v38) which is much better than the 650 I wanted at the minimum. Give it your best you guys and keep working hard!
Some additional info: I got a 690 on my gmatprep with 1 point higher quant and 1 point lower verbal. That 690 was my highest score on the official practice tests. I found Kaplan scores so be in the same ballpark as official tests but the verbal score is absolutely inflated. My diagnostic was around 450 (yup).