I too have read about the gelatin easel, but there is a better way (maybe not as tasty though..)
You can buy a can of spray adhesive from Gepe that produces a surface like that of a Post-It.
It'll last for weeks, is reported to be ph-neutral, and when you no longer need it, it can be rubbed off.
It just sounds more practical than the gelatine method.
BTW, you can use this stuff inside of a film holder too. But you have to cut out the tracks first.