Arrive early. The Party starts at 7pm but you can enter Magic Kingdom at 4pm with the party ticket. I suggest arriving at the Magic Kingdom at 3:30 so you are through security and ready to enter the park at 4pm.
Have a plan. Know what characters you plan on meeting, what parade you plan on viewing and where, and any special events you plan on seeing such as the stage show or dance party.
Get in a meal or rides before the party starts. If you arrive at 4 you can get in 3 fastpasses before the party starts, choose you first fastpass from 3:15-4:15, second from 4:30-5:30, and third from 5:30-6:30. If you get in the third ride at 5:30 then you can line up for popular characters (7 Dwarfs or Jack and Sally) early.
Meet rare characters. Many of the characters that are out during the party only are out at this time of year. Pick the ones you most want to meet.
Catch the Boo to You Parade and the Headless Horseman. I like to catch the second parade as it's less crowded.
If you have a Memory Maker get special pictures such as the hitchhiking ghosts.
Wear a costume. The Halloween Parties are the one time of year adults can wear costumes, about half of the people will be dressed up so you'll be in a good company.
Pick up some candy and treats. Grab a trick or treating bag and get some candy to eat while you wait in line for characters.
Pick a spot by the castle to watch Hallowishes, it is amazing.
Catch the Sanderson Sisters during the Hocus LocusVillian Spectular in front of the castle.