(In descending order of impact)
1. Intravenous Treatment
The doctor treating you in the emergency room doesn't have to sell you very hard on administering a drug.
2. Green Stamps
Executives suffer through long layovers to gain frequent-flyer miles. Here, the company rewards customers in a currency they care about.
3. Personal Relationships
The corner dry cleaner enjoys implicit permission to act in your best interest. A favorite retailer can "upscale" you (recommend something more expensive) without offending you.
4. Branding
Given a choice between the known and the unknown, most people choose the known.
5. Situational Selling
If you're in a store and you're about to make a purchase, you often welcome unsolicited marketing advice.
6. Spam
Where most marketers live most of the time: calling a stranger - at home, during dinner, without permission. You wouldn't do it in your personal life. Why do it to potential customers?