For me, the most important thing was to realize that the GMAT was an endurance test. After you take a practice test you will be tired. When you take the your practice test do it exactly like you would at the testing center. Give yourself the appropriate breaks and do the two 30 minute essays at the beginning.
I'm not too regimented in my studying either, so taking a prep course was helpful for me. Basically, it forced me to allocate time to study. When deciding what prep course to take, look at what kind of people they have as instructors. I ended up going to Veritas. All of their instructor scored in the 99th percentile on the GMAT. The classes are very small, so you get a lot of attention. They also give you access to 15 practice test (I think i took all of them) and tons of study material (the study material together was about 10 inches thick).
Another place i looked for more material was
http://www.beatthegmat.com/ . You will find even more practice material there.
In the end, if you put in the time and effort, you can do well on the exam.
I got a 720 (95th percentile).
I also see that you are thinking about getting a Kaplan book. I've looked at their material, but was not too impressed. They try to teach you little "tricks" to get better scores. Like on their reading comprehension sections, they want you to skim for content and structure, then read the question, and then go back to the article and look up the answer. The problem with this is that you don't understand the full context of the article or the question. I found it much better to do one thorough reading and then do the questions. I'm a pretty slow reader and thought that the Kaplan method was better initially, but it turned out that i was not understanding the articles and was missing questions because of this. The Veritas method seemed counterintuitive but actually worked better for me. Of course, your mileage may vary.