Stress and nerves prior to a checkride/stagecheck aren't uncommon at all. The instrument ride is a hard one period, whether stressed or not. I will not use the word failing for this post, because I do firmly believe that as long as you took away something educational from your 'bust' then it's not a failure, as much as a lesson learned. Example...I busted my multi instrument because I neglected to switch the NAV/GPS button back to Nav after flying the GPS approach and let nerves get the best of me. Instead of being smart about it and trying to sort out why VOR 1 and 2 were indicating something different, I let nerves get the best of me and blew through the LOC. That's the stupidest thing I could have done, I don't know why I did it...never did it before, never done it since. If anything, I've helped other people learn from my mistake...and I took away a lot from it.
Anyway, on to the case at point. As for helping yourself relax, to a point I don't think your ever going to completely calm yourself down. Some people may be able to walk into a checkride smooth as ice, but I for one was always nervous. One thing I found that really helped out, is to talk to the examiner in a non-checkride environment prior to the actual ride. This isn't to suck up or any nonsense like that, it's to remind yourself that he's a normal pilot as well. Before my private I walked over to the examiners hanger to meet him...he owned a chopper, single, and a multi. We got to chit chatting about his planes, he was also an MD-88 captain at the time. It really helped to settle me down and realize he was just another pilot.
As for the checkride itself, knowing the material well is definitely one of the hands down most important things anyone can recommend. Talk to people who are students, have them ask you questions and try to explain it to them. Talk to people who don't even fly. You'll learn quickly that once you are able to explain a topic to other, to yourself, or the wall...that you'll learn the material in a way that you never thought you would. Most flight instructors will agree, and that's why I think instructing is one of the best experiences any pilot can do for him/herself.
I am not good at small airplane systems. To a point systems are systems, but I am only familiar with the A-10/F-16...as I work phase and know both planes inside and out. I knew that was my weak area, and knew I needed to get stronger on it. I spent a lot of time in the hangar helping out A&P's and asking questions. As long as you show a general concern, and don't pester the hell out of them...they are usually very glad to help. Find your weak areas, and have people explain them.
Use acronyms. Aviation is all about acronyms. Brings a piece of paper with you into the checkride. I've never had an examiner knock me for writing out something so I don't miss something...especially for something like required items, or required IFR reports. There's a bunch, and with the nerves flowing...you can forget something. They know your nervous.
Have your flight plan wellllll planned out. Be able to explain your route, why you picked it, and throw in any needed information. I would always type up my weight and balance information, flight plans, etc. Made it easier to read, and more organized. When your organized, you generally feel more confident.
Go into the checkride organized. Have your FAR/AIM labeled for quick refererance should you need it. Most DE's aren't going to nail you for having to look up a thing or two...no ones perfect. Once a student, always a student. Your always learning, and will never know it all, even though a lot of pilots think they do. Have your maintenance records in order, and know where to locate the required inspections. I've seen a lot of CFI's wait until the day of the checkride to cover aircraft logs. Not the best time to try to cram a lesson in.
Lastly, just frickin' relax! lol. Bring in some water to drink. Your DE was once in your shoes, knows your nervous, and isn't there you fail you...even though he may have horns and a pitchfork - so it seems. Take your time answering a question, and if you really just don't know something...admit it. Don't make stuff up, they've been flying longer than you...and will know when your trying to BS them. Use it as a learning opportunity. I've learned so much on all of my checkrides, and don't regret a single one of them...even after busting the multi instrument. It's made me a much better pilot as a result.
Now go fly.