1. Your examiner is not out to get you: he wants you to succeed! (It's more paperwork for them if you fail than if you pass, believe it or not.) The goal of the oral is to make sure you have a basic understanding of the principles of flying. You learned them for your written, but this time the examiner will be checking to see if you're simply regurgitating pre-memorized answers or you actually have half a clue about being a pilot. (Don't worry: you'll develop the other half clue once you get your license.)
2. You won't know everything. The examiner won't expect you to know everything. You will, however, be expected to know where to look for the answer to the things you don't know. Looking up an occasional answer is fine, but if you have to look up every single thing, you're going to have a short day.
3. If it seems like you're getting grilled with question after question on a particular topic, that may be a good sign! What is likely happening in that case is that the examiner is trying to test the limits of your knowledge of your topic, and while you keep getting the questions right, they keeping asking harder and harder ones. Eventually you'll be stumped, but if you've really been getting drilled until you reach that point, you may have made your life a lot easier on the next. That's because the examiner will logically conclude that if your knowledge of that subject was rather deep, you're likely very well-studied and prepared overall and they won't have to push as hard the next time.
Let me give you an example from my own experience: during the oral for my instrument rating, the examiner kept asking questions about different charts and their symbology. I kept getting answer after answer right. After the 7th or 8th one in a row, it looked kind of like my examiner was getting irritated, ironically enough. Finally he asked me one that I didn't know, so I simply said, "Hmm. I don't know. Let me look over here." He said, "That's all I wanted to know: that you know where to get the answer." He moved on and didn't even wait for the answer itself! I had spent a ton of time preparing for this checkride, and the entire oral portion was 45 minutes because he gave me many places to shine and show off the results of all the hard work I had put into being ready for it. If I had had to look up every second or third one he asked about, I can almost guarantee that we would have been there a lot longer.
4. Don't just parrot the FAA-standard answer: understand what the technical goobledygook is trying to say. Going back to my instrument oral, one of the reasons it was so short was because if he pointed this symbol:
(Warning: not suitable for work!)
5. Project confidence in your answers. This is probably the toughest one of all to do, especially for your first oral. After all, you've never done this before and you're doing it "against" someone you hardly know. You don't want to come across as someone an insufferable know-it-all who isn't open to different perspectives or constructive criticism, but at the same time if you spend the whole time doing your best impression of Bill Lumbergh, you'll make life that much harder on yourself. After all, one of the purposes of the checkride is to evaluate whether you're ready to act as PIC: Pilot in Command, not Pilot in Umm, Well, Yeahhhh. Decide for yourself which of the answers to the question (which you're almost guaranteed to get), "What do you have to have for a plane to spin?" is better:
A. "Umm, well, I think you need a stall. Yeah, a stall. And I think there's something about the rudder. Like, I think if it's uncoordinated, it might help there be a spin. Right?"
B. "The plane has to be stalled. No stall, no spin. It also has to be uncoordinated; even if it's stalled, if the ball is centered there won't be a spin."
6. Know your POH. Unlike the written, where you were asked generic questions that apply to flying almost any small plane, your oral portion will contain some questions specific to the plane you'll be taking the flight portion of your checkride in. You don't need to be Angus MacGyver, A&P, but you do need to have a basic working knowledge of the systems of the aircraft you'll be flying once the oral portion is over so you can do some basic troubleshooting if something ever goes wrong in flight. For example, you won't be expected to draw the scary schematic of your electrical system from the POH, but you will need to know basic things like whether you have an alternator or generator, its output, whether you can switch it off and back on in flight, what systems of that plane are connected to the essential bus, etc. And know your fuel system! It's a very popular topic on oral examinations, and for good reason: a large chunk of accidents are caused by fuel problems, and many of those are caused because although there was fuel on board, the engine suffered fuel starvation because of an incorrectly-set fuel selector.
7. Have your maintenance logbooks tabbed, plus have a breakdown sheet of the required inspections written down with their dates on one sheet of paper. Use whatever system works best for you, but the goal is to be able to easily show that the required inspections and maintenance has been done and the aircraft is indeed airworthy. This is 15 minutes well spent, even if it means having to come in that much earlier on your big day (since very few places let the maintenance manuals out of their sight, so you almost certainly won't be able to take them home to do it ahead of time).
8. Here's another little item that will repay you in spades: do a weight and balance not just for your takeoff, but for your landing on your assigned cross country. You're already doing one for takeoff, and you've already figured out how much fuel you'll burn during the cross country planning, so it will take about 90 seconds to do the second one. Seems kind of stupid, since if you weren't overweight on takeoff then you surely won't be overweight when the plane is lighter after it's burned off ten gallons of fuel, right? Wrong. By doing both, you've demonstrated that you're ahead of the airplane and that you understand that a flight isn't over until you're back on the ground. Sure, almost all planes stay within CG as they burn fuel, but you've proved that your particular one does. It's probably the best 90 seconds of checkride prep you'll ever do because it's a tiny thing that says a ton about what kind of pilot you'll be.
9. All DPEs (Designated Pilot Examiners) are required to have a flight instructor certificate, and they are required to have given a minimum number of hours of dual instruction before receiving their designation. They don't mind teaching you something you don't fully understand, so if you don't know something, say you don't know. Naturally, if you say, "I don't know" to a lot of things or some simple things, you're going to get busted, but even a well-prepared person will have a few gaps here or there. Saying that you don't know and listening to what your examiner says is 100x preferable to trying to bluster your way through something you really don't know. They can tell, and they'd much rather see you be willing to learn than willing to lie about what you haven't learned.
10. This one is the second-hardest to do, right behind number 5: Relax! No one likes to have their performance examined, but your performance has been under a microscope (whether you've realized it or not) for the last several flights you've had with your flight instructor. If your instructor didn't think you were ready, they wouldn't have signed you off, because when they sign that final endorsement and your 8710, the FAA keeps track of their success rate—and if that instructor's pass rate is low, the FAA might have the instructor have to take another checkride of their own to keep their certificate. So believe me, if your instructor said you were ready, then relax and enjoy your big day because you're definitely ready!