I've been partaking in Photo-A-Day for several years. What helped me become an advocate is that I made my own rules. Instead of a photo-a-day, it's turned into photos. And usually those photos are centered around a particular theme. This year I've had the wonderful opportunity to be a part of Andre Gallant's Flickr group, where each week we have a different challenge. Often that challenge forms the focus of my shoot.
The challenge approach has really caused me to think differently about photography. It's stretched me, and sometimes made me feel uncomfortable about what I was shooting.
One example of this is "blurred image". In the beginning of the challenge week, I tried several different approaches to blurring an image. By the end of the week, however, I discovered how I could create a composition by using shapes and light. The initial thought of having a blurred image seemed to go against every single photographic principle that I embraced, until that moment when I realized how shapes and light can make so much of a difference. It may not be an award winning shot or series of shots, but it caused me to see my world and my photographic with a different eye.
Another example that really stretched me was making a long exposure where the exposure had to be greater than one minute. I have several NC filters and thought it would be an easy thing to do....not realizing that a minute is a long time for light to enter the sensor. It took many tries to get an exposure in mid-daylight that wasn't over-exposed. And it caused me to see what a truly long exposure might result in, if I give it a chance. I still have to play with this whole approach a bit more.....the key thing is that I am excited about exploring this new area.
So, I highly recommend the weekly challenge approach. Either using it as part of a group, or on your own. There are plenty of resources out there that can provide different challenges....you just have to jump in and enjoy!