Don't make it last; make people replace it often

A razor blade was something you bought once and used for a long time. A straight razor had to be stropped on a leather strap before each use and honed on a whetstone when it dulled. It took time to get the hang of it, and holding it wrong meant a cut. It was hard to do at home, so many people went to the barber for a shave.

King Camp Gillette was a traveling salesman for a bottle cap company. His boss, who had invented the bottle cap, told him to think of something used once and thrown away, so customers would come back to buy more. One morning in 1895, holding his dull razor, he wondered: what if only the blade were made thin and could be swapped out?

The problem was the blade. There was no way to put a sharp edge on thin steel. Metal experts at universities said it couldn't be done. After six years of searching, William Nickerson, an engineer from MIT, found a way to harden thin sheet steel and sharpen it. In 1901, the two founded a company.

In 1903, 51 razors and 168 blades were sold. A razor cost 5 dollars, half a worker's weekly wage. When a blade dulled, you swapped in a new one instead of reaching for a whetstone. The patent came the next year, and that year more than 90,000 razors and 120,000 blades were sold. In 1918, 3.5 million razors and 32 million blades were supplied to American soldiers at war. Back home, the soldiers went back to the store to buy blades.

You buy the razor once and the blades many times. This logic later came to be called the razor and blades model. Today capsule coffee machines and capsules, game consoles and games, printers and ink are sold the same way.

Worth adding: Gillette did not sell razors cheaply from the start. Until 1921, while the patent was in force, both razors and blades were expensive; razor prices dropped sharply only after the patent expired. The cartridge, two blades set in plastic, came out in 1971.

Instead of blades built to last, Gillette sold blades that are easy to use and throw away.
Rather than selling expensive, good products, selling cheap ones often paid off.