Some things I recommend bringing: a hairbrush (and if you have long hair, some hair ties or something to put your hair up/back with); a robe or coverup to wear during breaks; some bottled water or juice; some sort of a snack to eat during the session (if you don't eat a good meal before the shoot, I can almost promise you that you'll want something to eat at some point). Fruit, granola bars, pretzels and nuts make good choices.
Absolutely. If you want to bring someone, I do ask that you only bring ONE person, and that you choose someone with whom you really feel comfortable. My experience has been that friends make much better chaperones than spouses.
Avoid wearing tight-fitting clothes or underwear on the day of the shoot, and drink plenty of water. Elastic sometimes leaves marks on skin that can show up in the photographs. You don't need to do anything special with makeup or your hair unless you want to.
It's not a problem for me if it's not a problem for you. Strings can usually be tucked. Don't worry about it.
It happens. It's not a big deal, and I promise not to get offended, uncomfortable, or freaked out.