We've talked about tips for buying a touring bike, how are you going to get all of your stuff onto it? You'll need a few more accessories to go from townie to touring.
Racks are contraptions that you add over top of the front and/or rear wheels. They create a flat surface that you can strap bags/other items to and also provide the base on which to attach panniers. When you buy a rack make sure that it's strong enough for the poundage you plan to load it down with.
When you buy a bike for touring make sure that it has rack mounts for both front and rear racks. Front racks are certainly not required, but why limit yourself?
For cycle touring it's commonly recommended that you seek out chromoly-steel racks rather than aluminum (like the rack pictured above). The reason for this, according to MM (who is an engineer when he's not off adventuring), is that steel is not only stronger but also can be welded easily while aluminum cannot. That way your rack is less likely to break; and if it does you have the option of repair rather than replacement. Can you imagine being left to MacGyver a way to carry your bike bags without a rack? Yikes.
That said, like many people I am still using an inexpensive aluminum rack and haven't had any issues with it (yet!). Once day when I head off on a longer cycle tour I'll opt for an upgrade.
Panniers are bags that are specifically made to clip securely to your racks. Choose bags by deciding the carrying capacity your require, if you'd like them to be waterproof ($$), and making sure the clip system works for your bike.
In my experience Mountain Equipment Coop has the best deal on Panniers. Both MM and I have the ones below. While they aren't the fanciest bags you'll ever find, they are well priced and excellent quality.
Handlebar Bags are great for keeping a few valuables or much used items close at hand. While jumbo handle bar bags may seem practical at first, my experience is that carrying a lot of weight on your handlebars is annoying. In fact, many people may prefer to forgo this all together, which is a perfectly valid choice. I use a small, inexpensive handle bar bag and as I talked about here, use it to stow my valuables so that I can easily take them with me when I park my bike.
Trailers are an alternative (or possibly an addition to...if you have legs of steel) to panniers. Instead of carrying your gear in bags on your racks you can pull it in a trailer behind you. This is a pricier option but it does increase carrying capacity and allows you to haul gear on a bike not equipped with attachment points for racks. However, it's still not advisable to use a trailer on a carbon fiber bike. The torque of the heavy trailer could damage your light weight frame.
Fenders are covers that fit around your wheels, attached to the hubs and - on a road bike - inside the fork. Before I got fenders I didn't see what all the fuss was about, but MM insisted. Then, the first time I rode on a wet day after installing them I realized how wonderful they were. I remember instinctively bracing myself as I rode through a puddle, ready for the water to soak my shoes and...nothing! Not only do these babies keep you from drenching yourself, but they also prevent you from spewing a rooster tail of water at anyone who's behind you. How nice. They are fairly inexpensive and will make foul weather riding all the more pleasant.
Kickstands aren't just for kids bikes. A cycle touring bike without a kickstand can be a pain in the butt, however, a traditional "leaning style" kickstand wont do you much good when you're bike is loaded with gear. I recommend a pricey but oh-so-worth is double legged kickstand - I talked all about my beloved stand here.
Have you set your bike up for touring yet? What's your favorite piece of gear?