You may not be able to change your GPA or work history but you can dramatically alter your GMAT score with some studying tips. First, you should give yourself two to three months of studying time before you take the GMAT. In these months try to devote one to three hours a day to GMAT related material and five to six hours on the weekend. This time commitment may seem extreme but it will put you on track for a great score and will allow you greater confidence when you take your test.
In the very first week of your prep, you should familiarize yourself with the basics of the test: structure, types of questions, directions, time limits, scoring, etc. All these elements are important in helping you understand the logic behind the GMAT.
After you are familiar with the outline and elements of the GMAT you should assess where your skill level is. This way you will know how much you should be studying. Search the web for a free GMAT practice test, there are many out there and any will do.
After you know what score you have you will be able to pinpoint exactly what you will need to work on. Most prep materials recommend devoting five weeks to the verbal section of the test. This is because the math portion is relatively simple; you will never see calculus or advanced mathematical problems on the GMAT however, you will see verbal questions that you most likely would not have been familiar with without studying.
As you proceed through the studying process take a practice test every few weeks to see how you are improving. With great study habits and patience you will defeat the GMAT, good luck!