The first thing you have to try and do is relax. I would never send a student to a ride if I didn't think he would pass with no problem and I'm sure your CFI is the same way. I've seen too many students walk away with a pink because they just seized up. The checkride is as a much a show as anything else, you shouldn't be going unless you're a good pilot so this is about displaying to the examiner that you know your stuff. A few things to be sure you do:
-Make sure you go through the aircraft logbooks and flag all the items the examiner is going to want to see that way you don't fumble around like an idiot
-Have current charts (I know it seems obvious but I've seen this one before)
-Know your aircraft POH front to back. You don't have to have the whole thing memorized just know the important items and know where to find everything else.
-Know the big FARs and know where to find all the other ones without spending an hour going through everything
-Be familiar with the area over which you will be doing the ride. (Most examiners don't want to spend time going way out to the middle of nowhere, so know the local area) If you already know of some good places to do the ground ref it will help things go smoothly
-If you screw something up and exceed the PTS, and it isn't a safety of flight issue, immediately tell the examiner what you did wrong, why it was wrong, what you should have done, and ask to do it again. As long as you didn't scare the examiner he should give you a chance, after all it isn't a CFI ride.(<- not fun)
-Lastly (and I'm sure there are some things I'm forgetting) DO NOT BS THE EXAMINER, if you don't know something don't make up an answer. If you know where to look it up, then look it up, if not just say you don't know. The examiner mostly likely has flown in a month what you've flown in your whole life you can't fool him and he will know when you are BSing and will hate it.
Relax, prepare, and keep him bored in the air and you'll be fine