The Skill Test should be just like the mocks! Indeed there should be nothing in the Skill Test that you haven't practised before and nothing which your instructor feels you are not capable of. Put your faith in their judgement that you are ready for the test. They have seen many more candidates than you have!
The Skill Test is a practical demonstration that you are safe to be allowed to learn more about this flying process without the regular supervision of an instructor. If you fail, you need to view the failure as a safety net.
The best preparation is to prepare for the flight mentally. Go through the whole thing planning what you will do and when you will do it. The test is nothing more than a demonstration of everything you have learned to date. The examiner is looking to see that you know the right things to do, and that you are in control of the flight. If there is any aspect of your flying for the test where you are not sure what the correct thing to be doing is, ask questions of your instructor.
One thing I wasn't confident on before the test was diversion planning. If this is something you're worried about, get you instructor to setup a situation for you to give you experience of diversion planning in the air.
Don't worry about the speaking part of the test. You will probably be asked information that comes readily to hand. Make sure you know some the vital speeds for the PA28. If you've passed the aircraft technical exam recently you'll be familar will all the terminology, but it would be worth revising what is unique about the PA28. The oral test was perfunctory in my case, because I had done all my studying with that school, had only ever flown in the type of aircraft being used for the test and had been haging around the maintenance hangar talking to the engineers about Cessna 152s for months. I think the oral part of the skill test was put in to allow the examiner to check that the candidate knew how to operate all the aircraft systems before they got off the ground, especially in the case where the examiner is not familar with the aircraft, or indeed where the candidate is not familar with the aircraft that the test is to be taken in. This will not apply in your case. For my own interest I'd be interested knowning how many questions you were actually asked on the day. (Perhaps you could e-mail me)
There have been other posts on PPrune regarding test preparation, search them out and follow the advice, especially about getting a good night's sleep the night before.
Do relax as you will perform better than if you are stressed out.
Good luck.