A few days ago, a commenter asked about a good method for affixing DIY printed labels to jelly jars. Andy Biggs, mastermind behind the Beer and Jam Labelizers, mentioned he uses milk to glue labels to his bottles of homebrewed beer. What?!? After a bit of internet research, I learned this is a common method in the homebrew beer community. Labels need to stick on securely, but be removed easily so the bottles can be reused.
Must. Test. This.
Use whole milk or 2% to wet the back of the label. Then just stick it to clean glass.
(I've read that skim milk doesn't work well, but haven't tried it. It's probably similar to using just water: the label clings while wet but falls off when dry.)
If you need glue that's even stronger, try a recipe using gelatin.
Homemade glue ingredients:
2 tablespoons water
1 packet unflavored gelatin
3 tablespoons milk
Put the water in a small bowl and sprinkle the gelatin over it. Wait about 5 minutes. Then microwave the milk to nearly boiling and add it to the gelatin. Stir until the gelatin is completely dissolved. This recipe yields 1/4 cup of glue.
As you might imagine, the milk will spoil eventually, so the glue mixture will only last a couple days.
These tricks would work well for sticking posters to windows or mirrors, too.