At -40 C. In 50ml plastic containers in another Tupperware with silica crystals in the outside container. I try to keep track of what is there and label it well. If I think I am going to use something often I’ll keep a smaller container of them in the fridge as well. If I don’t have many seeds of something, I keep them in a drawer till I can make lots of seeds to freeze. I’m like Gollum with seeds. 
My bro stores them frozen in 5ml centrifuge tubes in a vacuum flask in the freezer, he reckons the vacuum flask prevents temperature variation if you are opening the freezer or transferring the seeds to another place etc. He’s probably right.
Freezing at least in my experience works just fine, I was shit scared about freezing my whole stash but from a germination test we did of 100 frozen vs 100 non frozen the difference was negligible if anything.
Try it with some crappy brick weed seeds as a test. It’s worthwhile.