See infomercials. All the effective ones say "but for a limited time only..." and "call while supplies last!" and "we can only guarantee this offer for the next 24 hours..." even though it's the exact same infomercial that has been on at the exact same time every night for a month. I've also never seen one of these products that wasn't "on sale" at some sort of "reduced" price.

They also usually include freebies "worth" hundreds of dollars with an item they are selling for less than $20. I've always wondered that anybody could be stupid enough to think that an item actually worth $100 would be included in a million $20 orders.