I argee with mostly all of what was said!
Joe Rocket tank bag! I have one and it ROCKS! Not too expensive, just get it online on EBay, you'll save a ton! They are black with 6 magnets and one strap. I NEVER use the strap and believe me I pack that thing to the brim sometimes! I've carried a w/e worth of clothes and my laptop in mine. Besides, when you pack it up real high, it makes a great place to rest your chin or helmet on when on the highway!
The JR bag also has a map spot on top where you can put a map or some people put their cell phones there facing up so you can see it ring and stuff. It also has a expansion zipper so it gets bigger. A extra pocket on the inside of the tank bag, kinda like in a suit case, the net thing on the inside of the lid. And it comes with snap on staps so you can carry it like a dufflebag. A nice hand carry handle and a rain proof cover that you can use. I've never used the cover, because the nylon material is pretty rain proof as well and it doesn't get that wet riding down the road sense it's behind your speedscreen.
Don't let anybody tell you crap about it being in the way for getting gas. Yes, it is but it's not that big of a deal. Take it off, put in on the ground in front of the gas pump, fill up, and put it back. I repeat...put it back on the bike!!!! I'm always worried I'm going to leave it behind!!
Tank Bags rock!!