Merge live with dev site

I have the standard local, dev, live site setup. So I develop on my local machine, upload to the dev site which is on the same server as my live site. After testing and making it is okay I will then update the live site.

It’s easy to duplicate the DB and the site files from the live site to my dev/local sites before I begin development. But I am wondering how you handle new changes to the live site.

For instance, you migrate the live site to your dev site. You then start making changes on your dev site. At the same time someone adds a new blog post or new content in the store on the live site. Now you have the new changes on your dev site that you want to migrate to the live site, but your content and DB are out of date.

How do you merge the changes from dev to live?