For a nickel

The current Fast Company reports that when Ikea started charging a nickel for shopping bags, consumption went down by 50% (95% in the UK).

Clearly, it's not the nickel.

The way you charge for something changes the way people perceive it. If the dinner special includes dessert, people get dessert because it's 'free'. Of course, it's not free. You paid extra for the special, remember?

A la carte pricing focuses your consumer. It forces them to make a choice in a spot where they didn't use to make a choice. It can highlight features that might have gone unnoticed (underbody salt removal treatment at the car wash, for example).

If you want people to notice a bit of consumption, charge for it. Even a penny.
If you want people to take something they had been leaving behind, give it away with purchase. Otherwise, they're wasting.

Here's one practical application. If you make something with low marginal cost like a CD, consider offering a second one (same title) for a nickel or a dollar. Why? Because if a customer buys a second as a gift, they've just helped you spread the word...