I was just wondering, as I have heard that if kept improperly it can become useless over night. Im not sure about that. But I am wondering how you keep it, and how long it keeps that way.
I can't imagine it would become useless overnight unless it was in a hot car in the middle of summer on the dashboard without tinfoil... just put it in tinfoil and put it in a cool place out of the light. It will keep for months without a noticeable decline in potency. If you're storing it for a long time, put it in an airtight container in the fridge or freezer and it will last almost indefinitely.
Oxygen, light and heat destroy LSD, so avoiding these three is key. If you are storing a sheet or more long term, I think the best way would be to vacuum seal in a storage bag, like a Seal-a-Meal type thing. Put into a lightproof container or envelope. Store in freezer. Should last years that way.