Some walk out if you decline the demonstration. If you say no to the high, inflated price, many carpet cleaners will call their boss and act like they're trying to get the price down as a favor to you. The price comes down, but it's still well above the coupon rate. If you insist on the coupon rate, beware the technician who cleans with water only, no detergent. Along the same lines, some cleaners will spray solution on your rug, but not extract it, leaving your floor soaking.
Many carpet cleaners charge more to move furniture.
And they won't move it back until you pay. Check your carpet warranty. Some warranties are void if you don't have your carpet cleaned in the first year or so. Get your carpet cleaned once a year. Vacuum twice a week.