MNSSHP is an absolutely awesome time! It was the highlight of our trip last year, and I'm hitting up 2 parties this year! I'd say do a quick search and get all the info you can, especially closer to the party so you know exactly what is where and who is out, etc. But for now:
-It's super fun if your party dresses up, I like to do it some kind of theme, and keep it Disney related (example, last year we had Tink, Peter, and a pirate. This year we have Alice & Queen of hearts at one party, and 2 Disney fairies at another one) you get a lot of interaction from the characters and cm's and it makes it unforgettable.
-You can get into MK with the MNSSHP ticket at 4:00pm, with the official party starting at 7. I usually aim to get there around 5:30, take advantage of a couple of rides, and then party.
-The Princes will be out with their princesses, in Town Hall theatre, the lines will be long, but it will probably be worth it (also note that Ariel & Eric are not out for parties)
-There are lots of characters out and about that are rare characters, ones you won't see otherwise. On the map when you arrive, you'll see the symbol signifying that there is a character there, but it won't say who. The disboards come in handy for this, and check, them out before your party to find out who is where.
-There is a special parade (with 2 showings) special fireworks, and a special stage show, they're all amazing and I suggest watching all of them
-Trick or treating trails are set up all around the park, definitely do them, but save them for the end of the night, so that you don't have to lug around the candy with you all through the party
-There are 2 dance parties, 1 in frontierland (last year it was in the Diamond Horseshoe) and 1 in tomorrowland (just infront of that stage area) they have characters in them, and are lots of fun. I'd say hit them up during one of the showings of the parade, when they're way less crowded!