DO NOT drink coffee or any energy drinks, it will raise your blood pressure aswell as your heart rate, those are the 2 things they will check first. and if you have a heartrate of 100 beats per a min and up you'll get DQ'd.
DO NOT suddenly remember being sick, breaking any bones, dental work or anything that is not already listed in the paperwork you filled out with your recruiter. If you do, your MEPS visit will end there and you will be sent home to get the proper paper work and waivers if need be.
say "yes sir, no sir or yes ma'am, no ma'am" to the people, military personel and ESPECIALY the Doctors, seeing how they will be the ones that will give you the Go or No Go.
You and about 12 other people will be in a room dressed down to your underwear where you will be asked to perform things such as the "Duck Walk", stretches (to make sure all your joints work properly) and things that may hurt or be uncomfortable such as walking on your Knees. my advise is if they ask you if you feel any pain or discumfort, say no.
I would go there with a list of jobs you'd like or wouldnt mind doing, in order from most disired to least. that way if they dont have the job you REALY want, you might be eligable for one you'd still be O.K. with. remember, after 2 years you may have the option to Re-class into something more desirable.
also, RELAX!