I was just invited to a conference — a very good one in fact — where the price of registration rises by $150 for every week that passes. This encapsulates at least two principles of behavioral economics. First, it combats our natural tendency to procrastinate. Second, if you register early you feel you have won a bargain when in fact it still costs something. This is of course also a planning externality if they know the number and nature of attendees sooner rather than later.