The only thing I have found which I feel is the best way to fight off the jitters is to know the material cold. That way, you don't even have a reason to be nervous, and essentially, the real GMAT should seem like a mere practice exam (like gmatprep) since you should have gotten very used to the format, timings, your shortfalls / areas to brush up on, etc. I would go in with the mindset that the real gmat is another gmatprep exam, except in a different location other than my desk @ home
Easier said than done obviously but under-confidence / nervousness should not hinder your progress in going to b-school.
I have been studying for the GMAT since beginning of June, about a couple of hours a day. I am planning to take the exam end august or end september. I am focusing on my weak areas and doing as many problems (some gmat problems, some refresher problems to solidify the fundamentals/basics) to build up my skills in those areas.
Another thing I noticed, which is odd, is that so many people take the GMAT "under prepared" - scoring 500s on CATS then being shocked when they take the real thing and score 500-540... What you put into it is what you get out of it, and I understand there are many reasons to go into the GMAT rushed (ie, work constraints, application deadline constraints, etc) , but I feel that it is a very dangerous thing to do. Next thing you know, they have taken the gmat 3 times and are scoring in the low 600s.
I simply feel that the GMAT is like a war and you have to go on your best day knowing you prepared the most for it and you did not leave any stone unturned.
fwiw,
- tsd