You already know the basics if you're planning on going to an invitational, make sure you sleep / eat / etc. As far as the tournament goes, hopefully you have, or are currently, testing the heck out of what you're playing against everything, play against tier 1 decks and tier 2 decks at a minimum enough times to know which decks are going to be easier to win against -vs- which will give you a hard time. For the decks you struggle against, play like, 10 times more games against them (Unless it's crystal clear the match is unwinable 90% regardless of what you do, sometimes that's part of magic)
In testing, you also have information available, seeing all the recent top 8/top 16's etc, you know what % of the field is likely to be or not be as you go in, and prepare for that, but know that magic is still magic and sometimes, you might be at the odd end of match up's at the end of the day. (Play enough tournaments, and sometimes you just get paired against crazy stuff more than once or twice in just a single day, it happens)
When you are at the tournament itself, you should be ready for the game, ready for the metagame, be confident in your deck choices / sideboard because you've done the homework and you KNOW FOR SURE that statistically your sideboard is correct against the average field, after that, you have no control over your pairings. I find it absolutely insane when I go to a large event and people ask to test a few games because they want to try something out.
Also, go in with realistic expectations. There are 100's, sometimes 1,000's of people at these tournaments, and know that when 400 people walk into a room, statistically by the numbers, only 2% of 400 are going to top 8, 1% of 800, etc... When I go in, my first goal is always "Just try to stay alive long enough to top 8" then when some 2nd loss hits me and I know I'm X-2, it becomes "Ok top 16" then when another loss hits me, it becomes "Ok lets top 64/try to get money/walk out with a winning record/etc..." Knowing this, again if you play in the same event 10 times in a parallel universe, you will very likely not have the same record in all of them, so when it's over, as long as you know you did what you could and did correctly, you should be happy, and also know that magic is a very intensive game, and mistakes are going to happen regardless win or lose.
I was lucky enough one time to win a large event a couple years back, and even then I can think back to that same tournament, round 6 against Merfolk, and I made a play mistake that could have costed me from even making top 8, but my opponent didn't notice so I got lucky. (I cast a lethal Spell against my opponent with NO MANA open & NO LANDS UNTAPPED and my opponent had out a Cursecatcher, could have used it to sac and I would have lost, a bystander pointed it out to both of us after the game was officially over.)
Oh and on another note, never be afraid to call a judge, judges WANT YOU TO CALL THEM, I have an insane respect for judges, they are there by VOLUNTEER which I didn't know for the longest time, and they LOVE what they do as much as the players playing the game! Call a judge for ANYTHING (As long as it's not ridiculous of course, common sense there) and any judge will come right over to help.
On less serious notes, if you're going out of town for an event and it's not somewhere you've been before, and you're going with a group of friends which is the norm, get together with everyone before the tournament and say this. "It doesn't matter who makes it, all that matters is that one of us does, if someone in the group top 8's, then we can celebrate together and it will be an awesome weekend!"