Setting out the ingredients and letting friends play mixologist helps keep everyone happy; the host can stay out of the kitchen, and guests can commune with their own signature version of the drink.
Clear part of the table and build some levels. Throw a cookbook, pot or two under a table cloth, and you have instant platforms on which to group the ingredients (this is for presentation as much as practicality -- the different heights make it easier to see and grab the goods). Have a bucket of good ice and a stock of pint or highball glasses.
Vodka. It's not necessary to go top-shelf. If you really want to mix it up, offer a mid-level gin or tequila, as well.
Purists can make their own (see recipe below), but there's nothing ignoble about using a favorite bottled mix. If you're feeling generous, offer a few, including an extremely spicy one for the self-flagellating-est of your friends.
Clamato, Tabasco, Worcestershire Sauce, and tomato juice.
Here's where you can truly go nuts. Offer a platter of celery, lemons, dill pickle spears, and cubes of cheddar cheese, but you don't have to stop there: The Bloody Mary is an excellent staging ground for just about anything you want to add to your morning meal. Try out artisinal pickled beans or peppers, onion slices, or roasted garlic heads.