Rather than having the photos go offline at a certain date, you should raise the prices. Sorta like the 20% off within 30 days idea, but in reverse. (which saying 20% off the first 30 days might actually get people more excited than saying prices go up later =P)
Also, make sure you are watermarking the images. In the Smugmug forum there was a debate about watermarks, and one person noted that when they enabled them on their gallery they got a few emails from people asking why they could no longer print the photos from their own computer. Especially since you are shooting for free, they might have the misconception that they can print them for free..
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