We don't live in a world of limited shelf space.
And that's a good thing.
In Chris Anderson's book The Long Tail, Revised and Updated Edition: Why the Future of Business is Selling Less of More he opens up with a story about an older book, almost out of print, whose sales were suddenly revived by its proximity to a newer book covering the same topic.
Recommendation engines in online bookstores picked up on the buying habits of its customers and began to pair the two together. People bought in this manner and an infinite loop was created. Over time the older book began to outsell the newer book.
It's a great story that illustrates something that most of us are already coming to terms with. We no longer live in a world that shuns older work. Not entirely anyway.
Because of the internet and it's ability to bring older, relevent, content to the masses on a daily basis we have opportunities to sell things beyond their launch date. This is true whether you sell fine art prints through your online gallery or whether you have past client work you want to show off as proof of your work.
When we blog, tag, and push our content out into the Internet we create little bits of information that is waiting for discovery. Take a look back on the work you've done in the past. Is there a way to push it back to the front of people's minds?
If your older work is still relevent, if it's still timely, then you are allowed (encourage really) to tweet about it.
Look through your archives. I'm sure you'll find a goldmine of work that is evergreen. Push it out there. If it needs updating then do so. Then push it out there.
As a photographer I take a look back at some of my older photographs to see if I've missed something meaningful. Especially if it's something I hadn't shared before. When Adobe releases a new version of Lightroom and they've tweaked their processing I like to look into those older photographs and see what the new filters can do with them.
Take one day a month to comb through your archives (whether that's your art archives or your blog archives) and see what can be worked on, improved upon, and pushed back into the streams of your followers.