It's easy to be so focused on creating new content all the time that you forget about older posts, but the truth is that your old posts can be an amazing tool for attracting new readers as well as keeping your current readers on your site longer if you know how to use them.
First, really look into your analytics to determine which pages are getting the most organic search traffic (primary dimension: landing page in Google analytics), and then which pages/posts are getting the most views overall (content: site content: all pages in Google analytics). Even if the pages/posts that rank highest in search engines aren't the most popular posts on your site, they're still worth paying attention to, because they're a great chance to attract & keep readers, especially if you notice that you're getting a higher percentage of new visitors to your site through these pages.
Once you've determined the pages/posts you need to work with, here are some ways to bring them back to life:
Create a New Post
Use an older popular post to inspire a new one. If your “Spring Trends under $100” post from 2010 is still getting page views and search engine traffic, perhaps it's time to create a “2013 Spring Trends under $100” post. If you don't have a “related posts” plugin so readers will see similar previous posts at the end of your new post, make sure to link to older similar posts – this keeps readers on your site longer.
If you have a post that is very popular already, and is easy to update/add to, this is a great option. My post on where to buy the Alexander Wang Rocco bag from 2009 is the most popular page on my site after my home page and I update it frequently. When I do update it, I make sure to note that in the title of the post, so in search engines visitors will see that it's been updated recently, and not just the original publication date of July 2009. This is really useful for shopping posts; perhaps you did a generic “Favorite white tees” post a year ago, update it now with new favorite white tees, or mention a few more you'd like to try…
Share it with other Bloggers
If it's a really old post, first, update it with some new information, then submit it to Links a la Mode if you didn't originally. Also, if you see that another blogger has written something similar, or you think your post might be interesting for her, send her a link (DO NOT LEAVE IT IN A COMMENT!!) and ask her if she'd share it with her readers if she found it useful.
Add it to your Sidebar
You can do this either with a popular posts widget, or alone. I create graphics for my most popular pages and link to them prominently in the sidebar. Make sure you have images associated with your popular posts; pictures are worth a thousand words!!
Promote it on Social Media
There are always opportunities to attract new readers, especially via social media. Make a habit of tweeting or posting on Facebook one of your most popular posts every week (or every day if you have a lot!). Encourage discussion around them also, you may get good ideas for a new post inspired by the older one. Also, whenever you update a post, make sure you promote it! Since it won't go out in your RSS feed of new posts, make sure your readers know you updated it with new information; I like to use my weekly newsletter for this.
Have you used older posts to breathe new life into your blog? How have you done it? Have you found it useful?