We're going to talk about the best way to test mods. Whether you're beta or alpha testing for another modder, or just for your own mod, it's important to follow at least some of these guidelines to minimize your chances of destabilizing or even ruining your game because of a bad mod.
If you're the one testing your own mod or distributing it to be tested, make sure you clean your file with TES5Edit and generate an SEQ file (if necessary) first. Many issues I have had and struggled with for hours have been due to missing one of those steps. So don't make the same mistake I did.
A New Save Game
Every time you test a mod, you want to create a completely new save game. This means creating a new character. But it does not have to mean going back through Helgen. You can use this Alternate Start mod to skip that entirely, since going through that entire sequence each test would be ridiculous. Or, (thanks to Ddmlink for pointing this out), you can use the console to "coc" (syntax: player.coc locationeditorID) to a location from the main menu. But make sure you create a new save - it may take extra time, but, while not fail-safe, it will lower the chances of ruining your saves dramatically.
Make sure you have no other mods loaded other than the file to be tested and its required masters. Unless, of course, you are purposefully trying to test the mod's compatibility with other mods. But then, only load the mod to be tested, its masters, and the one you are trying to test its compatibility with.
Backup, Backup, Backup! (And Backup)
You need to back up your entire Skyrim folder before installing the mod to be tested. For modders especially, it's a good idea to back it up fairly regularly, but for testers you should definitely do this. This is a backup of your entire Skyrim folder before it was even touched by the tested mod. If you have issues with space, then you can compress the folder. I recommend using 7-Zip for this, since it has the highest compression rate out there and it's free.
Write Down Notes
This has less to do with stability then giving good feedback. If you see anything you thought was interesting, and especially any bugs or issues, write it down in a text editor or on paper. That way you won't forget something. And it's not being critical to report a bunch of bugs - the more you find and report, the more bug-free the tested mod will be.