Here's my experiece with storage.
DMT seems to be very stable. I left a small dose on top of my old cannabis growing cabinet for six month. This was during a extremely hot summer and it was sometimes 45 degrees C in the cabinet so you can imagine the temps on top of it. After six months it was very active.
That said, usually I store DMT in the freezer. I just put it in a petri dish, put that dish in a plastic bag in the freezer. (I also put it in a box to keep it dark all the time). This seems to be the best way I heard. Only thing to be careful with is taking out a dose. When you take the dish out of the freezer, you have to let it cool down to room temp before opening it.
L&V
HC