How do I merge changes from a development copy of the site to the live site without losing new content? What is the best procedure for merging work done on a development copy of a site to the live production copy? Often times there has been a lot of new content added to the site since development started on the newest features. And most additions to a site will involve database changes. So copying any new files is easy, but what about the database? How do you merge your changes with the existing production database without losing new content that was added since the last time you updated the production site? Are there any modules that help with this?
For content types, views, and structure changes on the dev site look at using Features to export the database to code.

For content migration there are many options, but not a single solid solution. One example is the Deployment suite.