All kinda depends on your circumstances.
I got put up in a hotel with an enlisted Marine recruit who snored like a motherfucker...kept me up till 3 AM with a 5AM wakeup call.
Probably why I failed my depth perception test there, but did OK on my civilian retest.
Hydrating supposedly helps for the vision portion as well...but more importantly, urinalysis. I took a piss b/c I didn't know better when I woke up.
So I arrived at MEPS pissed out, dehydrated from a PRT, on 2 hours of sleep, and first thing was urinalysis. Having no "ammo" I had to drink nasty ass tap water by the cupful for half an hour before I was ready to go. Point is...make sure you'll be able to piss when you get there.
The underwear thing is essential...but for everybody else's sake, wear boxers. The prior service 30+ guy with a beer gut and tight white briefs traumatized everybody there during the duckwalk type stuff.
I'd always been told for any blood tests to fast for 24hrs prior, so I did that as well. But apparently it's not an issue since they let people eat.
And when you get out, look your file over and make sure you're cleared for flight duty. Let your recruiter know if anything "questionable" came up...they can clear you for a general officer physical, but obviously the aviation aspect adds a few extra requirements. If anything you're not sure about came up, ask your recruiter...I probably could've saved about 3 weeks if I had.