The "Real" Rules for Buying an Amp
Jump on the flavor-of-the-month bandwagon. It'll give you bragging rights (for up to 30 days, but you must act now). Besides, chicks dig guys who have flashy new amps. Guitar magazines, internet discussion forums, and mass-marketer catalogs are all excellent places to find thoughtful, unbiased recommendations. Avoid stores that specialize in used gear - they only sell that crap because they can't get the best brand-new stuff.
Insist on expensive, esoteric materials and construction techniques. (An astute listener should be able to identify the brand of power supply filter caps just by listening to a few well-chosen notes.) Failing that, find an amp that's older than dirt. (Authenticity is important here, if you can prove that the dust inside the chassis has been undisturbed for forty years or more, so much the better. To find a qualified appraiser, check your Yellow Pages listings under "Carbon Dating".)
Get an "improved" version of a design that was originally conceived to maximize profit, because the "good voodoo" in maximizing profit definitely improves the amp's sound. This is why boutique copies of classic amps are so expensive: the profit has been maximized twice.
Your best choice of amp will, of course, depend upon your style as a musician. If you're an afficionado of Texas blues who plays the occasional local jam, you should insist on finding a low-powered amp with tons of headroom. Your dream amp will be able to get singing power-stage distortion at conversational levels and still have enough headroom to hang with an aggressive drummer.
But everyone knows that blues is for old guys who couldn't cut it as a rock star while they still had their youth and virility... Younger players need a more modern amplifier designed especially to complement those one-finger power chords and the clever and highly original clean-verse/dirty-chorus arrangements. If your normal playing posture has your picking hand somewhere between your thigh and your ankle, you should definitely look for an all-tube amp with lots of chrome and at least one separate 4x12 cabinet to help you crank out the bone-crunching low-end that lets the drummer and the bass player know who's boss. And don't forget channel switching: two is good, three is better, and four is obviously best. You'll need footswitching, too... those controls on the guitar are totally useless.
(Bonus) If all else fails, follow the artist endorsements. Artists only endorse amps after conducting extensive personal research of all the available alternatives. The most famous artists featured on the largest media outlets all have the highest degree of personal integrity, and offer their support to a manufacturer only because they think the product is the best they've ever seen.