I've done several of these for celebration events and boy are they alot of work, but it is alot of fun and sure is worth it.
First off: Determine the number of people that you plan to serve. Allow 1 1/2 lbs. of carcass weight per person. This will tell you the amount of pork to purchase. To estimate the cooking time, allow 1 hour per 10 pounds of pork. Ask the butcher to remove the eyes and have the pig ready for roasting when you pick it up. This mean they'll clean it for you. There's nothing worse than a stubbly dinner. Do not exceed 225º F cooking temperature for the first two hours of cooking. After that I still stay under 300. Cook it skin side up for the first 2/3 of the time then flip it over for the remainder. A wire bracket can help this process. An internal temperature of 170º must be reached. Have additional coals started outside the grill, ready to be added as needed, to maintain the proper temperature. As the pig nears doneness, place a meat thermometer, or two of them to be certain, in the center of the "Ham" of the pig, making sure not to rest the thermometer against any bone. When the thermometer registers 165º to 170º, your pig is ready to transfer to the carving area. Let the pig rest for 20 minutes before carving.
Other points of interest:
Use wood chips, you deffinately want to smoke this.
Season well. I like doing my pigs simple Garlic Salt and Pepper, lots of it, inside and out.
As the hog cooks I spray it with cider vinegar, oil, and garlic salt.
Note you can use any other, and there are many, seasoning and spray/mop combinations you want.
I enjoy placing either an apple or and empty beer can in my pigs mouth for asthetic purposes, but then I also name the pigs too.
Toast the hog with your favorite beverage before serving.
Remember the process and the celebration of cooking a whole hog is the fun part.
You should get many many other ideas on this topic, everyone has their own tradition/favorites when it comes to roasting hogs.
Good Luck and Enjoy!