A good tank bag has magnets in the middle and two or four 'arms' with magnets on them. It stays very solid on the bike, and often also has a clear vinyl top to place maps and such behind (convenient for DVD rentals, actually). You'll want to keep your interac and credit cards away because those strong magnets could kill them almost instantly, though some people report that they're perfectly safe; do so at your own risk.
There also exist strap-on tank bags that don't use the magnets. They're not nearly as convenient, but they hold on solidly. A couple of folks have reported magnetic bags blowing off in strong crosswinds, so a strap-on pretty much makes that impossible. It's highly unlikely that a magnetic bag will blow off, though.
Marsee, Joe Rocket, Tour Master, Nelson-Rigg and many others make excellent tank bags. These are available from very tiny size (big enough for a baseball) all the way to "massive touring" size, which obscures all view of the speedometer and tachometer. The Tourmaster Magnetic bag fits well and holds lots of stuff:
A tank bag is a good choice if you only have a bit of stuff to carry. A reasonably sized tank bag will fit a sweater and rain jacket, or binoculars and a camera, and so on. A carefully packed larger tank bag can hold enough clothing for a weekend trip. It's also great for everyday use, in that when you get where you're going, you can easily pop it off in just a couple of seconds and take it with you. Same for installing it.
Be cautious not to let grit accumulate on the bottom side that touches the tank, and also be sure nothing gritty is on the tank prior to installation; otherwise, scratching can occur. Some have reported adverse effects from leaving a bag on a tank for long period of time (many days or weeks), so don't do that.