This is one way to do it:
If you want your LMDE to track Debian Stable, you need to make a slight adjustment to your sources list.
1. Click the shield icon in the panel.
2. When the Update Manager opens, click on Edit > Software Sources.
3. Click on the Third Party tab and highlight Debian Testing.
4. Click the Edit button, change “testing” to “squeeze” and click OK.
5. Do the same procedure with the Security and Multimedia repositories.
6. Click on Close and refresh the Update manager.
You should now be tracking Debian Squeeze when it switches from Testing to Stable.
** If I missed a step, someone please let me know.