1. Choose an "open" theme
Your party theme will set the tone and help guests pick out costumes. Avoid being too specific ("Colonial Victorian London circa 1853") or guests may not participate. Some good examples include: The 70s (disco to hippies), the 80s (goth to prep), Hollywood (glamour to movies), Villains & Heroes, Scary (monsters to political figures).
2. Send timely invitations
Halloween is THE party night of the year, so make sure you get your invitation out to your fiends at least 3-4 weeks in advance. (Evite is the leading authority on guest lists so use it if you have more than 10 guests.) Also consider having your party a week early, since more warm bodies will be available. Be clear on your invitation about location, time, and any special expectations (bring a dish to share, bring your own booze, no rugrats under 4 feet tall).
3. Plan the menu
A complete meal or appetizers and cocktails? Balance sweet and savory, hot and cold, meat and vegetarian. For larger groups, set up a buffet. If you plan on having a potluck, assign specific items rather than categories (i.e. instead of saying "dessert", say a cake, or a mincemeat pie, otherwise you'll end up with 14 plates of brownies). Think food safety and consider how you will keep hot food hot, and cold food cold. No one wants to leave your party in a body bag. Also, Halloween food should be tastefully gruesome (or nobody will eat it).
4. Delegate the small stuff
Have a fiend who wants to bring something but can't cook? They can bring plates, napkins, or soda. Have someone else in charge of the ice. Fiends who want to help can arrive an hour early and set up prepared foods or the bar. Don't count on extensive help. Most guests usually want to remain guests and enjoy themselves.
5. Have a game plan
Know what is getting made the day before, the day-of, or being purchased that day. Set up serving areas, and have all furniture positioned the day before. Have serving dishes out and ready to be filled. Make checklists and actually check them. And don't forget to clean the bathroom!
6. Create a flow
Rather than setting up all the food on one table, create stations: set up the hot food in one area (closest to the kitchen), the cold food & chips on the table, the bar outside (since it's typically the messiest station). This will help create a flow for guests to wander about and explore.
7. Arrange various spaces
Provide a quieter retreat for fiends who want to catch up. Others will want to hit the booze and music – and then there are the smokers (set out ashtrays). If you have a room off limits (to harbor rabid pets or heirloom family jewels) cordon off or lock the door. Police tape from the party store in a zigzag pattern works great.
8. Attend to hosting duties
First and foremost, you should enjoy your party, but do not neglect your hosting duties. Make sure guests have food and drinks, circulate and spend time with each friend. A drunk host is really no fun (someone has got to keep it together). Also, cook hot appetizers in batches allowing late-comers equal opportunity to the food selection.
9. Take photos
Halloween lends itself to great pictures. Have a camera mounted on a tripod set to take cool nighttime photos. This will capture the environment well. Consider distributing 3 or 4 disposable cameras – they aren't just for weddings. Throughout the evening ask different guests to make rounds, taking pictures randomly.
10. Deal with antisocial
Fiends who hide in dark corners are especially good at helping the host during the party. They prefer tasks to sitting alone and rotting. Give them a tray and have them pass out hot appetizers or cocktails. This will create instant likability and your wallflower fiend may even mingle. Circulating the food also ensures that all the food is eaten. Some guests may not visit the food table, but will eat if it comes to them.
11. Prepare activities early
If you are planning a wicked activity, gather all the necessary items ahead of time. It's no fun trying to find candles and eyes of newt in a dark, guest-filled house. Place all components in a box and stash it under the food table, closet or other staging area where it will be undisturbed until needed. Keep in mind some people will not want to participate – don't force anyone to play wicked games.
12. Make good riddance favors
A small favor in a little trick or treat bag will send guests off with a smile and finally get rid of last year's Halloween candy. Personalize the favors with guest names. If small children were excluded from the rituals, be sure to prepare special goody bags for their parents to bring home to them. It shows appreciation for asking parents hire a sitter, who is now likely dead.
13. Ending the night
While most parties fizzle out naturally, at times some guests linger. Finding a tactful way to say the party is over is challenging, but the task should not be awkward. Say something like, "thank you so much for coming", hand them their coats or a favor; ask if they've had a good time and what they plan to do tomorrow; turn on all the lights (if dark, or vice versa); turn off music; begin cleaning up or vacuum loudly. Guests should be safe to drive, otherwise arrange for a ride or call a taxi.