Now I'm not suggesting or encouraging anyone to reuse their masks as it's stated 'discard after use' but if you really want to reuse your sheet masks, here's some tips I've learnt from reusing mine.
1. Keep the essence and sheet mask separately in the fridge. If you store them together, the mask will soak up all the essence and when you place it on your face, the mask will feel dry.
2. If you're worried about the cleanliness of the mask, wash it with clean water and do NOT scrape the used essence back into the packet.
3. Slather the essence all over your face then place the sheet mask over it. If you still have a bit of essence left, you can also put it on top of the mask to make it adhere better.
4. If the essence is very very watery and there's a lot left, you can use those 'pill' compressed masks instead if you don't like the idea of reusing the sheet.
5. If you find that there is insufficient essence, I think it'll be alright to reuse the mask over your own homemade masks. I've personally done this over my yogurt mask as it was too watery and the sheet helps it stay on my face instead of dripping onto everything. By reusing masks, you can cut your spending by half. However, I must admit that sometimes, the mask isn't that effective anymore during the '2nd round', especially the Watson's energizing eye mask. There was almost no tingling sensation that I had felt on the first use.
** Please reuse masks at your own risk. I cannot guarantee that any bacteria picked up from your face will not brood in the sheet mask and cause outbreaks when you reuse it. Lol