Firstly, you forgot to mention when do you plan to take the exam ?Because going by your scores, you may need to improve substantially to be around the 700 mark.
1. Do the concepts well and then do sufficient number of practice problems from a particular concept type.
2.No point in taking the tests at the moment and your stagnant score very much show why ?
3. Write down a plan for yourself which will act as the guiding force for you.
4. Maintain an
error log. I cannot overemphasize the importance of this
Since,as you said that you are good at Maths and just need to brush up,here are some of the recommendations for Verbal
SC :
MGMAT SC Guide,
Aristotle SC GrailI think after going through both the books ,you should be good at SC
CR : Powerscore CR Bible. Better than
MGMAT CR Guide
RC :
RC99. A fantastic book with 99 passages on the GMAT pattern.
Apart from this, practice all the questions in the OG12 ,and if time permits in the previous OG editions,from each of the areas
I think by following a disciplined approach, you should be able to improve