In my opinion, the ETS Physics GRE Sample Tests are very important in your preparation efforts, and I believe they warrant a special discussion on how to effectively utilize these resources. These sample tests allow one to get a pretty good idea of what to expect on the exam day and therefore they provide excellent preparation material.
It is recommended that you take the sample tests under simulated GRE conditions. For example, you will want to give yourself the allotted time which to complete the exam and you will only want to utilize information and resources that will be available to you during the exam. During your sample tests, as well as during the real exam, you should not spend large amounts of time on any one problem. The goal is to maximize your score and the easy problems count just as much as the difficult ones. You do not need to answer every problem correctly to get an excellent score.
If it is early in your preparation efforts, you may be able to justify slightly relaxing some of these requirements. For example, if you know you need a certain equation to solve the problem and don't yet have the equation memorized, then it may beneficial to allow yourself to look up the equation so you can work through the problem. Of course if you do this then you should make sure you memorize the equation later and perhaps even go through its derivation.
As the exam date nears however, you should probably take your practice exams under identical conditions. This would involve the necessary time constraints and lack of access to formulas. It would also involve bubbling in the answers, working with a pencil, no distractions, and the other detailed factors involved. Perhaps you will want to be very meticulous and see what type of desk you will take the exam on. If it is one of those miniature desks then you will probably want to get use to the hassles involved before the day of the exam. Also, find out if what you want to bring to the exam is allowed, such as certain beverages, ear plugs, etc. Anything you can do to familiarize yourself with the exam day will serve to minimize the stress.
At some point after the practice exam, you should deffinitely go through the questions you didn't get to, couldn't answer, didn't answer correctly, and perhaps even the questions you got right. Some people find it very beneficial to thoroughly analyze the ETS sample tests.
For more thoughts on the very important process of learning from practice problems I suggest you read the discussion - Working Practice Problems.