First off, make sure you get some live poker experience. It can be very strange if you've only played online, there are rules that apply in a live card room that you may not be aware of (we have an article on our site about playing live for the first time and some of the rules live card rooms have).
Get there early and soak up some of the atmosphere, check out the rooms where you will be playing and make sure you know the rules about seating and breaks, etc, before it all starts. The more relaxed and familiar you are with the whole thing the easier it will be.
If you have played live before you've probably never played for the extended hours that (hopefully) you will be there for. Drink coffee (not alcohol) and plenty of water, also eat well but not too much when you get the chance. Also lots of events have massagers which work in the room while the tourney is going on, I'd recommend trying this if you start to feel uncomfortable and restless.
If you speak to big event pros they'll say the most mistakes are made towards the end of the day, it can feel like a marathon sat there for so long. To try and keep your concentration, maybe put in some pre event training online (play some 8 - 10 hour sessions).
Good luck.
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