Read English-language newspapers / magazines every day (15-30 min). New York Times, Wall Street Journal, Scientific American, The Economist. Have a dictionary available while you read.
Do you also have our SC book? Or some book that teaches you the grammar you need to know for the GMAT? (Make sure it's a GMAT-specific book. You need to concentrate on the things that are tested by the GMAT.) If you don't, get a book that will teach you SC.
These articles linked in this post can help you to learn more about how to study:
http://www.manhattangmat.com/forums/articles-on-how-to-study-figure-out-weaknesses-etc-t9230.htmlIt's better to do a little bit (1-2 hours) every day than to study for 5 or 6 hours all on the same day. You can follow the course syllabus found on this page:
http://www.manhattangmat.com/freegmatlearningforum.cfmIn the "Highlights" section, download the Official Manhattan GMAT Course Syllabus [pdf]. This is the full syllabus for our regular course; if you're studying on your own, you can still use this syllabus as a general guideline, even if you don't have access to all of the resources listed in the syllabus. Just do what you have.
The syllabus even lists when to take the practice tests. After you take a test, read the "Evaluating your practice tests" article (one of the articles at the first link, above) and use the information to help you evaluate your own test. Use your results to help you develop a study plan for the next few weeks until you take another practice test.
And, of course, use the forums! There are a lot of problems and explanations already posted here. You can also use us for help on strategy questions, as you're doing right now!