First of all, good luck! Remember that your examiner wants you to pass. You have to give him a reason not to pass you. Make sure you read through the PTS and that you are familiar with all of its contents. The things they will always ask you about are airspace (ie requirements for flying in each, wx minimums), charts (do your best to memorize the chart's key. Also, know how to tell if an MOA is in use or not), basic aerodynamics, basic systems, PPL limitations (ie passengers cannot pay more than their pro-rata share), XC navigation, and other things. If you do not know something, and I can't stress this enough, DO NOT MAKE SOMETHING UP. It is 100000 times better just admitting that you don't know than to venture a wild guess. As long as it is not a piece of basic information that you should have memorized, the examiner will let you look it up in the FAR/AIM. Make sure you know how to look things up in the FAR/AIM. Having post-it notes in the book for regulations pertinent to the PPL would be helpful. If you look at the beginning, it will have a listing of important regulations pertaining to each level of pilot license. You don't have to flag all of these in your book, but some of them would definitely be worth putting a post-it note in. I strongly recommend buying one of those PPL Oral Exam Guides as they give you a very good idea of what to expect on the oral. If your instructor hasn't done so already, ask him/her to do a few practice oral exams with you.

As for the checkride, remember to always do clearing turns before each maneuver. If you aren't set up for a maneuver, don't start doing it. Just let the examiner know you need more time to set up. Also, if you do mess something up, it's not the end of the world. Tell the examiner what you did wrong, and they may let you repeat the maneuver (as long as it wasn't God-awful). If you stray a few feet out of the PTS required performance for a maneuver, don't freak out. Just make sure to correct yourself ASAP, and everything should be okay as long as you aren't doing this on every maneuver. Try your hardest to treat the flight like any ordinary flight with your instructor. I know this is much harder than it sounds, but the more you treat it like a routine flight, the more you'll relax and the better you'll fly.

My PPL oral was about 1 hour, and it wasn't tough. I must admit that the only checkride I ever failed in my life was my PPL, and it was because I didn't do a clearing turn between a power-on and power-off stall. I was devastated, but I elected to continue the checkride and knock off all the other items that needed to be accomplished. My re-check took 20 minutes, and I have been a clearing-turn Nazi ever since.

Let us know how it goes!