creative commons - flickr.com: http://www.flickr.com/photos/digitaleye/2289436931/Enlarge Photo
DO get winter tires, not snow tires. Snow tires give you some sort of nubby tread pattern that's better in the snow, but it doesn't assure a softer, grippier compound that makes a difference on ice or hard-packed snow. Some of the least expensive snow tires might actually be worse on glossy, packed snow than all-season radials, because of their lack of special siping to help channel surface water away. Tires sold for winter use often carry the M+S designation, but you want the extra winter-tire rating—marked with a "mountain snowflake" symbol—indicating it's a true winter tire with those more advanced compounds and design attributes.
DO get a cheap set of steel rims with your winter tires. Don't expect to mount winter tires on expensive performance rims. You don't want salt and sand grinding into them, or salty slush getting in and corroding them. If you have them mounted on another set of rims that you don't mind getting grimy, it makes swapping them out at the end of the winter for your shiny summer alloy wheels a lot easier and cheaper.
DO mount winter tires on all four wheels. Some tires stores won't even allow you to buy a partial set of winter tires, for good reason. Buying snow or winter tires for just the drive wheels is silly. It can cause the vehicle to be dangerously unstable in corners or during hard braking, and in newer vehicles with stability control and anti-lock brakes, you might be compromising the effectiveness of those systems.
DO swap them out as soon as spring arrives. The softer compound used in winter tires wears down much faster on dry tarmac—especially in warmer weather. So as soon as you're quite sure the snow and ice isn't coming back, swap them out for your summer set. Stow them in the garage, and you'll have them all ready to go by the time next winter rolls around.