Waterproof your kit. Ortlieb or Lifeventure dry bags are amazing. Your sleeping bag needs to stay bone dry at all times, or you're going to have an absolutely dire expedition.
Take baby wipes for personal hygiene. Get someone else to wipe down your back.
If you have waterproof trousers, roll them up and store them in the rain cover compartment of your rucksack if it has one, you'll always need to use them both at the same time.
As far as food goes, it's really your preference. If I'm looking for something to munch on whilst on the go, I take a bag of jelly babies with some non-salted nuts thrown in.
Don't take too many clothes. I take two sets when I'm on a multiple-day expedition. Dry kit for sleeping and hanging around camp, and wet (dirty) kit for walking in. Clean socks are essential though, take plenty. I pre-talc a pair of socks, ball them together and stuff them into a ziplock bag.
Keep hydrated, keep your spirits up AND the spirits of those in your team, be prepared to slog it out, and you'll have a great time.