You should not do as many PrepTests as you can from now until October 1st. This is counterproductive.
Schedule the four TestMasters diagnostics for every Saturday, starting from August 20th. Take the week of Sept. 11-17 off, because you'll have the TestMasters Jam sessions. Then the following Saturday TestMasters will adminster June 2005, which is one week before the actual exam. Between weeks, review the diagnostics thoroughly.