Do not use this tutorial without adapting it to current situation. I'll provide a new tutorial once LMDE 3 is released and will write it such that it is more future proof than this tutorial has proven to be.
Sometimes you may need a newer version of an application than is available from LMDE 2 or an application might not be available on LMDE 2 (like firejail for example). In these cases you may consider installing the application from Debian testing. You can look up which versions of a package are available in Debian here https://tracker.debian.org/.
The way to safely add Debian testing is to configure it so that it's APT Pin-Priority is between 100 and 499 (inclusive). By lowering the APT Pin-Priority we make it so that only when you explicitly state you want to install something from testing will it get installed from there. Once you have installed a package from testing it will also be upgraded from there by Update Manager. You also get security upgrades for those packages. Though note this is handled differently: https://www.debian.org/security/faq#testing
Proceed at your own risk! Adding Debian testing is not recommended but if you are diligent in checking what apt says it will do when you install an application from testing and make sure it only installs that application and nothing else then it is unlikely to cause problems. It is fine to install a few applications from Debian testing if you are careful. This is testing though, so versions from it may have issues. At any time you can downgrade a package to the LMDE 2 version as shown below.
Step 1: Add the repositories to your system. Create the file /etc/apt/sources.list.d/debian-testing.list. On Cinnamon and MATE you can do that with the following command (replace "gksudo xed" with "kdesudo kate" for KDE):
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gksudo xed /etc/apt/sources.list.d/debian-testing.list
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deb http://httpredir.debian.org/debian/ testing main
#deb-src http://httpredir.debian.org/debian/ testing main
deb http://security.debian.org/ testing/updates main
#deb-src http://security.debian.org/ testing/updates main
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gksudo xed /etc/apt/preferences.d/debian-testing.pref
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# To find this information for other repositories use:
# egrep '(Origin|Label|Suite|Codename)' /var/lib/apt/lists/*Release
# Meaning of the fields:
# o=Origin, l=Label, a=Suite, n=Codename
Package: *
Pin: release o=Debian, a=testing, l=Debian-Security
Pin-Priority: 300
Package: *
Pin: release o=Debian, a=testing
Pin-Priority: 300
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Package: *
Pin: release o=Debian Backports, n=jessie-backports
Pin-Priority: 400
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apt update
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apt install -t testing package
Downgrade a package installed from testing: To downgrade a package you had installed from testing you should first check if originally the package came from LMDE 2 or from Debian stable. The most fool-proof way is probably to search for the package name on this page: http://packages.linuxmint.com/list.php?release=Betsy. If you found the package on that page downgrade it to the LMDE 2 version with this command:
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apt install --reinstall -t betsy package
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apt install --reinstall -t stable package