The cheapest option
Just use a duffel bag and a bungee net, but...
...if you have a little money, you might consider something like an Eclipse Rumble Pack tail bag. It's 12x11x7" high (expandable to 12") and fits nicely on the rear portion of the 250 seat. It has an adjustable bungee cord attach system and the four hooks fit perfectly on the 250 bike hooks.
Eclipse makes another nice one called The Fast Pack, and Nelson Rigg, Tour Master, Ocelot and Lockhart make some good ones, too.
Or you can make one yourself using a soft-sided cooler
It's an Eddie Bauer 18-can cooler from Target; regularly ~$30. Buy a $5 grommet kit at Wal-Mart (camping section) and install four grommets in the bottom four corners. Kit includes all you need; you basically punch a hole, insert top and bottom grommet pieces, and hammer them together; pretty easy. Then, use some small bungee cords (about 8 inches long and 1/4" in diameter) as follows: fold in half, stick loop through grommet (loop to the outside) and connect them in two pairs; one pair in front, one pair in rear. Hook them together with a short segment of coat hanger in the middle (if they're not quite long enough without it) and crimp so they won't come apart.
You can also get some cord clips (these were $1.25 at REI) to prevent the cords from creeping back into the tail bag when you aren't paying attention. If you measure carefully, you should be able to place the grommets so the clips don't touch the paint on your bike.
The finished product
Of course, there are many variations on this theme. The bag below was found at Wal-Mart for about 10 bucks. It's a fishing tackle bag, brand name "Okeechobee Flats" (so don't go looking in the automotive section). It has six major storage areas, including an inside pocket (great for map storage). There is a mesh pocket on the outside that may be good for rain gear. It offers good flexibility for the price.
The bag looks to be made well and semi-waterproof. It comes in various colors. You can attach it to your bike using the same method as the soft-sided cooler (above).
And there's always the Rubbermaid Action Packer
Payne has inspired some (quite a few, actually) members to create a lockable, waterproof trunk for occasional runs to the grocery store and for storing helmet and cycling gear when hiking, picnicking, etc. Pretty simple.
It's a 56 liter Rubbermaid clone attached to the passenger seat, with bungees mounted inside the trunk and with U-bolts holding onto the grab bar for extra security. Wrap the U-bolts with electrical tape and put cloth patches on the trunk where it contacts the grab bar to keep either the U-bolts or trunk from scratching the grab bar.
This takes 2 minutes to put on or take off the bike. Attach the U-bolts, hook up the bungees, and away you go. It's got combination luggage locks that keep casual meth-heads out. A determined thief wouldn't have any problem just cutting into the side of the thing with a big knife.
On a test run to a distant grocery store the trunk stayed very secure, even in a downpour on the interstate. The paper grocery bags were completely dry. This thing doesn't have gaskets, so in case of a prolonged downpour, things might get a bit damp.
The Action Packer should be available in many hardware or other building/contractor supply stores. Here is a list of known suppliers, as of early Fall, 2006:
But wait, there's more. Much more.
This is the 24 gallon model, purchased at Canadian Tire. It fits a jacket, pants, boots, gloves, helmet, and spine protector... all of it, with room to spare. You could even fit small mammals in there if you wanted to. Our rider claims the the box is actually no wider than most shoulders with gear on, so it doesn't affect visibility.
Ventura Rack Option
A rack on the back can make organizing a little easier. You might want to consider the Ventura rack. Here are the directions, to give you a little more information.
Ventura rack in FAQ