First off, hang in there. A PRB doesn't have to mean the end of things, but can actually help to determine how they can better help you to get through the classes and get better for flight school.
Now, as for the board itself, brutal honesty! Seriously. The biggest thing that will turn board members off (I know from being one), is you trying to BS them. If you are struggling, ask for help. Tell them exactly what your study habits are, why you think you are doing poorly and if they ask, what you think they might be able to do from the instructor side to help. Now be careful with the last part. That is of course only if YOU have put in 110% and need a little extra assistance. "I need extra class time because I go to the beach every afternoon to chase girls" is not the right answer! I would suggest taking a hard look at your study habits and come up with a couple ideas of your own of how you think you can get better. Obviously what you have done so far is not working, but if you show up with at least part of a plan of how to change, they will see that you really give a damn and be much more willing to work with you.
If you are doing everything the Navy is asking, you are studying really hard, paying attention, and then just don't get it, no one can fault you. And if that is the case and your 100% isn't good enough, then don't worry. You gave it your all and that's all they can ask. From past experience I imagine that if this is the first board, you should be fine and given another shot.
Good luck and study! Keep us posted.