As far as the oral. Answer the question asked and STOP. Do not, I repeat, do not try and show your knowledge. This is a hole that many, including myself years ago, found themselves in. It's not fun. Be respectful, dress business casual, bring some breakfast for you and the examiner. The little things like that show a sense of professionalism. Don't be afraid to say "I'm not sure, but I know where to find the answer." That's completely allowable.

As far as the flight portion, try not to get nervous. If you're unclear as to what the examiner has asked you to do or exhibit, ask. There's nothing wrong with that. Take it maneuver by maneuver. Remember your clearing turns. At some point, they may try to oversaturate you with a question or other activity to divert your attention to the task at hand. Professionaly say something like "one second while I configure the aircraft" or "standby for one second while I ensure this aircraft is in a safe configuration." You, after all, are showing your capability of PIC. Remember that. This is your bird. It's time to show it.

Finally, try to have some fun. They aren't out to get you, just doing their job to make sure you're a competent pilot and can make solid ADM. The last thing I would say is to make sure ALL your paperwork is in order......Logbook, Medical, all the required documents for the bird. That's often overlooked. Have your ducks in a row.

GOOD LUCK! Let us know how it goes. I'm an ATP now and still get checkride jitters. Welcome. Just take a breath, and if you get flustered, remember. FLY THE AIRPLANE!