Well, you've learned what
not to do...
Seriously, and I assume you want to do well on the real deal, the first step in preparation is to set your mind to the task. So, think of the matter like so -
The preparatory exams you use are meant to get you ready for the GMAT exam.
Each of them is unique, and essentially has no value once taken, except as a teaching tool, so you want to take them in the same serious manner as you will the formal test itself. A practice test, once taken, will show you where you are strong, and where you need work. Many students compile an
error log to track where they need to improve. The best way to get an accurate assessment from these tests is to take them under real-world conditions as best you can. Four hours, stopping for breaks, trying to get them all right, and answering everything, including the AWA portion, even though that's not part of the formal score.
The GMAT is the sole standard examination accepted at major business schools, and can make
all the difference in getting accepted to your first choice ... or even your second and third. Competition to get into a top school is very, very tough, so even a small difference in your GMAT scores can decide whether you are in or out. While grades and your resume and essay and reference letters will be factors, for many schools you won't get a second look unless your GMAT makes the cut.
The school you go to will determine the prestige of your degree, which in turn will determine what sort of positions, companies, and pay you may expect after graduation.
So, looking at the chain I have laid out here, you should be able to see that even these practice tests have a place in a crucial life decision, one of the biggest you will ever make, and not something you should be careless or casual about.
I don't want to stress you out, but it's important you understand why this is important. Good luck.