I have never been before so I know absolutely nothing. I don't even know what questions to ask.
Your party ticket will get you into the park at 4pm even though the party starts at 7.
The headless horseman makes his ride before the parade. My favorite spot to get a picture of him is between the Emporium and Barber Shop. The picture in the first post was taken there.
The parade is a not miss in our book.
The character M&G are really fun. Just be aware that lines for the more popular characters can start over an hour before the party starts.
There are two character dance parties. Last year the one was villains themed and the other was Monster U themed.
Trick or Treat lines can get really long, but if you aren't into seeing characters, the best time to ToT is at opening. Start at the Splash Mt trail, go to the POTC stop, then the Tiki room, then take the passage way to Frontierland and make a ToT stop at the dance party, and stop to dance with the characters. Skip the ToT line at the Steamboat, because lines for that are shorter later. The candy trail between New Fanasyland and Tomorrowland has one entrance and one exit area. If the entrance is in New Fanatasyland, stop at Pinochio Haus for ToT, then head to the trail. The candy stop at Stitch always has a long line for some reason? Go to the Tomorrowland ToT locations and stop at the dance party to dance with the characters.
Hallowishes is one of the best firework shows Disney has to offer. To me, this is a not miss. Our favorite viewing is between 7DMT and BOG. You are surrounded on all sides by fireworks. It's a very intense experience. The show is designed for best viewing from Main St though.
If you want to see characters, you'll need to prioritize who you'd like to see and plan accordingly. I'd suggest getting in line no later than 6 for Jack and Sally or the 7 Dwarfs.
ETA: Please keep in mind that it is nearly impossible to do everything the party offers. Decide what about the party is important to you.
That's all I can think of off the top of my head. I'm sure other people will chime in with their tips.