
Could you eat all your food from within a 100-mile radius of your home? these guys did, for a year, and their website has tips for how to eat local (and a contest for the best 100-mile thanksgiving menu).
If the average food ingredient travels 1500 miles from the farm to your plate, local food saves 1400+ miles of fuel and pollution. Local farms are more likely to be smaller family farms, many of them organic. in most cases, it's easy to meet the farmer or visit the farm.
They have a tool to show you a 100-mile radius around your home. I have it easy here in Ithaca; my radius includes most of the farmland in upstate NY. I can even get beer and wine without leaving the city (though I admit I don't know whether the beer's ingredients are local).
Ithaca's farmer's market, which just closed for the winter, has an impressive variety of food grown within 30 miles. This past summer I signed up for a CSA with Early Morning organic farm - I paid my share in advance, and for the whole season I could walk up to their stand at the market and pick the veggies I wanted.
This is unusual for a CSA, though. Typically after paying your share, you pick up a biweekly or a monthly box of produce (or meat, or whatever they offer), packed for you at the farm. I've just signed up for a winter CSA of this style from Blue Heron Farm. During the winter months they'll bring me potatoes, carrots, and other veggies that are either harvested during the winter, or that were put into storage after harvest.
Eating locally is easy in summer and fall (remember all those local, antique apples I was raving about?) but winter and spring are harder. I'm not planning on going on a strict 100-mile diet (if I were, I'd have gotten into canning) but Ithacans can shop at Greenstar, which is proud of the large variety of local food they carry.
UPDATE: Here are the winners of the 100-mile Thanksgiving contest. The winner was from Syracuse, not far from here.