Pretty much what everyone said...relax. It's so important because your really taking so much information and cramming it into 40 hours of sim/actual IFR time...once you get out there and start flying in actual IMC more often, things will fall together. Read through the ASA book like everyone mentioned, and there's a few good FAA pubs. Know the FAR's that apply to instrument flight. I had to draw the different approach lights on a board. Outside of that...just a lot of cut and dry IFR stuff with some scenario based questions thrown in.
Oral wise - I got drilled on approach lights, FAR's, approach plates (know all the oddball symbols), charts...he didn't get into anything aeromedical with me for obvious reasons (Dr.)...did cover some weather, but not much. Every DPE is different in what they like. Some of the DE's we had liked systems on the IFR ride, I think I only got asked two...my oral went 6 hours though. This guy was anal about the oral. Other guys were out in 45 minutes but had a rougher flight.
As for the flight, yeah definitely relax. The day of my ride we actually had to postpone because the wx wasn't going to allow for us to get the non precision approaches in...once it improved enough to get the NP approaches in we went. A little additional pressure I guess...taking your checkride in solid IMC. I just went into it thinking hey, this is what I trained for...if I can't do this now, will I be ready to do it single pilot? We did actually break out for a while. However, I did make a really stupid mistake that I still kick myself in the butt for everyday. And the ONLY reason I didn't catch it, is because I started getting nervous...it was the last approach, all was going well, and I knew my husband was returning home from Iraq that day and I was scheduled to go shortly thereafter. I let my mind slip for half a second, kinda lost track of what I was doing and made a mistake that I wont relive again - done it enough here. lol. Had I relaxed and analyzed the situation, I wouldn't have done that. Flown that approach way too many times to do something so stupid. Just keep your head in the game...the DE wants you to pass - well so they say. lol. Key is to put all outside stresses to the outside, and concentrate on flying. They don't mix well with a checkride, I learned that quick.
You may not fly the best flight of your life, but examiners expect you to be nervous. And you will be, rightfully so. No matter what the outcome is, you will eventually walk away as an instrument rated pilot. As long as you take away something new from your experiences, then the flight really was worth it. Learn from your mistakes...and enjoy being able to show off some newly gained knowledge.
Best of luck and let us know how it goes!!