We picked up some sand dollars from the beach when we were in California for our anniversary and we brought them home with us.
We decided to do something with them to preserve them, and after looking around on the internet for a while, here is what we are doing:
First we soaked them in regular water for a while to get the dirt and sand off of them. We kept changing the water until it didn't get cloudy anymore.
Then we added some bleach to the water, and let them soak in the bleach/water mixture for about 10 minutes.
Then we rinsed all the bleach off, and set them in the sun to dry, and to finish whitening up.
Finally, we brushed a coat of regular white glue mixed with water on them to harden them a bit.