Step 1: Identify Constraints
1) How many people will attend?
This is the big one. If you've got more attendees than tables, chairs, plates, silverware, etc then you'll have to devise work-arounds. Tables/chairs can be rented, borrowed from friends and neighbors, or improvised from milk crates and pieces of plywood or spare doors. If you're short plates and silverware you can decide to use disposable or ask guests to bring their own, picnic style. Make your own cloth napkins to be extra-cool.
2) Who's making/bringing food?
Identify what you'd like to make, and what (if anything) you'd like your guests to bring. Make sure your guests are on the same page with this.
3) Buffet or table service?
If you've got more than one table a buffet is pretty much required. It also reduces the need for small, passable bowls which would otherwise require refills. You can put the whole dish on the buffet and be done with it.
4) Storage space
How much space do you have in your refrigerator and freezer? This determines what you can make ahead. If you live in a cold area an enclosed porch or deck can supplement your fridge/freezer space; if it's too warm out you can use an ice-filled cooler to bulk up your refrigerated space.
How many ovens/racks do you have? You'll need to block out time accordingly, and choose oven vs. stovetop preparations as necessary.
What time do you plan to serve dinner? This one is under your control; pick a time that will allow you to wake up at a normal time and still be able to run everything through the oven with time to spare. Schedule this one for your convenience.
Step 2: Create Attendance List
I noted whether the guests were staying here for the night, what they were bringing (food, beverages, air mattresses, etc), and any food requirements (vegetarianism, allergies, etc).
Step 3: Create Gantt Chart
Col. 1: enter all of the food you intend to cook, and the major infrastructure preparations necessary.
Column 2: break down each item into its major steps.
Columns 3-n: list the available days, and assign each step to one of these days. Consider fridge, freezer, oven, and stove availability when assigning a prep day/time. You may choose to break the final day's activities into several columns, or add text to your boxes to clarify timing for high-use resources such as the oven.
Column n+1: identify the cooking or serving dish you'll need. If you don't have enough, borrow or use disposable pans. Running out of pans can be a serious problem.
Column n+2: identify the course. This helps in timing and food placement.
Columns n+3/4: after dinner, recap your quantities and evaluate what you plan to do next year. I wish I'd done this last year; since I didn't have good records of what was produced/eaten/desired from 2005's 27-person dinner I frequently over-estimated for this year. It's always better to have too much food rather than too little, but a bit of optimization here can give you more room to maneuver.
Hopefully you've got helpers. Give everyone their own color, and label the blocks that must be done by a specific person. Mark up-for-grabs tasks with the "anyone" color, and give the oven a color for good measure. It's not a person, but it's time is valuable and requires scheduling. Black out the squares as you go.
This means that anyone who wanders into the kitchen wanting to help will be able to check for open jobs, and hop right in without too much prompting.
Step 4: Run With It
People seemed to love checking the Gantt chart for open projects, and fought over who got to black the squares in. It was lovely, and seeing the columns go dark makes it feel like you're accomplishing something.
Since we had so many dishes in the works, it was nice to double-check against the spreadsheet. Forgetting the stuffing in the downstairs chest freezer or the trifle in the back of the refrigerator would have been sad, but was easily prevented by comparing against the master list.
Step 5: Enjoy the Party
Again, I cannot emphasize enough: make sure you take notes on this year's successes and failures, particularly with respect to quantity. You'll appreciate it next year.
Also, if you felt forced to cut corners (using disposables, faking tables/seating, etc) now's your chance to plan ahead. Craig's List and eBay are your friends, but it's even better to build things yourself. There are nice instructables on woodworking, severaldfferenttablesandchairs, creativeservingdishes, aprons, napkins, and the like; try them out over the next year. If you build anything cool document it- we'd all like to leverage your experience.
Instructables has a boatload of useful recipes as well; try them out and add to your repertoire, and share your favorites with us. Best of all, for a potluck you can send recipe assignments with an accompanying Instructable to make sure things go well.