GMAT Strategy

Hi everyone,

I started my preparation in September studying all the weekends (avg 5-6 hrs per day). So I can say I have been studying for 20 days. I did the OG12 almost twice, the Kaplan once,the PS OG10 once and the GMAT Verbal Review once. I took 1 GMATprep at the beginning and I scored 640 then I did 1 GMATprep last week and I scored again 640. In the meantime I did 3 Kaplan tests and I scored alays 550 ( I found these tests more difficult).I am planning my GMAT in March/April 2012 (aiming at at least 720) and from January I will add 1/2 hours in the morning before starting working.Do you think it is a feasible plan?I think I ll buy some Manhattan books as well. Do you have some other books to recommend?
Thanks in advance for your help
Best,
Ale