As someone who's quite heavily tattooed, the above advice is sound. Don't have any booze, you'll bleed loads and lose half the ink, leaving you with a rubbish finished piece. Take water, lucazade, and a mars bar with you. Eat and drink throughout - the adrenaline will do all sorts to your blood sugar and body temperature, and you don't want to pass out! Take something to keep you occupied - I played Angry Birds and read a book at my last sitting, it'll distract your mind. I find that watching makes the pain easier to bear. I can see how much they've done, how much is left, and how it's coming together, which is fascinating to me!
Lastly, don't go thinking its going to be unbearable pain. Pinch the place you're being tattooed - top of your foot isn't so bad. That's the max pain you'll feel, plus some stinging to begin with, before your adrenaline kicks in. Expect it to tickle on your feet, my feet twitched like mad when I had mine done. I always go with the mindset of "It's gonna hurt a bit, there's no two ways around it, but it'll look good after, and my sitting is only an hour or so long. It's worth it!"
I must warn you though, the day after is when it'll hurt you most. I can only describe after-tattoo pain as feeling like your skin has been burnt and shrunk. I couldn't walk for 2 days without it hurting like hell. My feet swelled up and I could only wear sandals (in winter!) that left my whole foot exposed. Worst bit of the whole experience IMO.
Just keep thinking its worth it after at most, a week. Then it'll be healed and look brilliant.
Make sure to moisturise, but don't use Vaseline or Savlon. They'll pull the ink right out.
Hope this helps, sorry for the essay!