I'm in a similar boat!
First time(early October), tried to allocate 2-4 hours per week to study, about 3 weeks of studying, got 690(don't remember the break down).
My prof and recommender is the head of the department at a top Ier school, and basically says that a strong GMAT score(his number was >760) would give me a "good" chance at his PhD program. But of course, if I could walk on water, then there is a "good" chance that I am a supernatural being. (yes, awkward phrasing, there are better options for the sentence, sorry been studying too much for the sentence correction - my weakest point).
My strategy has been to study almost every material I can get my hands on - princeton review
, kaplan 800
, GMAC 3 practice exams, etc.... All the material have a slightly different focus, and the actual GMAT(and to some extent GRE, SAT, etc...) always seems to have some minor material that catches me by surprise. I generally do pretty well in the math sections(800s on GRE, SATs, yada yada), but last time, there was a problem that took me some 10-15 minutes, which I eventually gave up on and selected something to go on.
My exam is on saturday of thanksgiving, and I'm taking the whole week off to study, relax, focus, and recooperate. I find that rest and focus has the largest impact on my performance on ETS tests.
Anyhow, Good luck!
(I've taken, in chronological order, 9 SAT 1, 3 SAT II, 3 GRE, and 1 GMAT. ETS is making a small fortune from me).