The best source to practice for GMAT is Official Guide. I am from Engg background and was working as business analyst so I didn't have to work at all for Quantitative section but my English was terrible. Hence, spent most of the time on that.
Here is what I would recommend for people who are thinking about preparing for GMAT:
1. Take the practice test 1 that you receive from GMAC after registering online. Actually giving the practice test without any practice gives you the confidence that Oh! I could have got this much without any practice whatsoever.
2. Pick up official guide and start solving questions for each topic very sincerely.
3. More important than solving questions is reading the solutions. Even if you get an answer correct, read the solution with same diligence.
4. Follow the online forums for GMAT preparation like gmatclub.com, beatthegmat.com. pagalguy.com ... and many others. I personally was a big fan of beatthegmat.com and I tried to solve questions from every topic on this site (atleast for 2 hours everyday during 15 days that I prepared for GMAT in total)
5. You can subscribe to test series like Manhanttan or Kaplan during this time and keep giving tests every week or so ( I gave tests every 2nd day since I had only 15 days to prepare for)
5. Last step and most important - Give your GMAT prep tests 2 one day before you final test date. Generally the score that you get in this test is exactly the same score you get in your GMAT. (I got exactly the same)
Final advice - don't waste your money on unnecessary resource because Official guide is the best.