A number of top restaurants (otherwise booked, on a Saturday night, for 2 weeks) auction off a single table, where 50% of the proceeds go to the restaurant, and the rest to a foundation for gastronomical research.
Another approach is to require a credit card and charge almost the price of a full dinner if you don't show up. The 41 in Barcelona goes one step further and makes you pre-purchase the entire meal.
Some guests will spend another $300 on an expensive bottle of wine. I think it's fine to let some people pay extra in advance to be guaranteed to get a table.