While practice tests are important, you don't need to take one every other day. The students who improve the most tend to do fewer problems, but spend lots of time analyzing them. This is especially important in verbal. Do all the verbal problems in the OG and truly understand why every right answer is right and every wrong answer is wrong.
Also, start reading any well written material in English that you can find. Books, magazines, newspapers, whatever... just get used to reading a lot!
450 to 680 is tough, but I've seen it happen. Study smart, not hard, and visit the forum frequently!
Jim S. | GMAT Instructor | Veritas Prep