I've said this before and it still makes sense to me. United should weigh not only bags before boarding, but passengers as well - and charge extra for pounds over a predefined limit. They could even give you a discount if you're less than a certain weight (kids, Kate Moss, etc).

New York City is thinking about taxing sugary beverages to incentivize habits that lead to better health. United should promote lower jet fuel consumption (and hence increased profit) by providing similar incentives.