How should I store unused stickers? What is the best way to store stickers for future use? I have some unused LEGO sticker sets in plastic bags. The one on top in the bag has become brittle and is flaking.
Think about what causes the stickers to deteriorate: 


Heat loosens the bonds in the glue, which results in stickers peeling.
Exposure to natural light will cause the colours in the stickers to
fade over time. Unlike LEGO bricks, stickers fade faster under natural light.


With those points in mind, keep stickers in a cool, dry, place away from sunlight like in a cupboard or drawer. Zip lock/food bags are also helpful for organising stickers which is why they can also be used to store instructions and catalogues. 

Stickers transferred from the sticker sheet onto other surfaces will also be affected by dust, oil and any other particles that will affect the lifespan of a sticker. Make sure the surface is clean and dry before applying stickers.

You can use a hairdryer to easily remove stickers from bricks.