Come on Leopard.
Dont say you failed for a good score of 660. I understand you may have been targeting different score, but I believe 660 is not a FAILURE.
There are people who have gone from 500s to 700s....so if you try hard its not at all impossible. In fact I believe anyone who scores 600s can at some point achieve 700..but whether efforts required are justified or not is something individual has to decide.
Out of SC and RC, SC is easier to improve. RC can also be improved but it takes longer.
You may find some tips in my writeup
here and many such writeups on "Share your GMAT Experience" forum
In short
RC - Gin's notes for RC + LSAT Superprep + LOTs of PRACTICE
SC - Manhattan SC +
OG + 1000SC(of you have time) + GMATClub verbal forum.
For SC one thing I noticed is if you are picking up an answer without actually applying rules then it is possible you will miss that same question next time you do it...if thats happening then you will never score too well on SC. Its good to do guessing and POE but in exam you will see lot of questions with 2 answers very close and if you dont know rules then you not only have 50-50 odds which always work against you but also have risk of wasting too much time deciding this or that. (do last 30 SCs from OG11 with timer on , you will see what I mean)
Also NO Practice without timer since you already know basics.
If you are targeting 700 then you have to consistently score 30-40 points above that range as indication of you being ready for the actual exam.
Take a week off , rejuvenate and kick a$$.