For that reason, keeping readers interest is extremely important. There are, of course, numerous ways to get people to visit your site, but this idea is a really fun one. Not only will they be logging in to enter the contest and logging in again to see other people's entries, but they'll also be smiling when they do so. Who doesn't want their web site's readers to feel great?
One way to accomplish this is to host a lolcat contest. This involves posting a picture and then asking your readers to submit their own suggestions for a caption, ala ICanHasCheezburger.com. The lolcat phenomenon is well-known throughout the Internet, so you will have to do a minimal amount of explaining.
I've seen these contests garner dozens of responses, and everyone involved has a great time. Why not give it a go on your organization's web site? While using actual cat pictures is fitting for organizations like the ASPCA, you might want to consider something that ties in more closely with your own mission. Just remember to keep it lighthearted and fun.