This article is from the Toys FAQ, by firstname.lastname@example.org (Scott Gordon) with numerous contributions by others.
From Eric G. Myers (email@example.com):
"Far and away, I recommend a product called "Goo Gone." Its
available at retail outlets like K-Mart. It can be found in the
cleaners section or sometimes in the automotive section. It is a
citrus product and will not damage the card if used correctly (do
not over-apply and allow to soak into the card). Others will tell
you to use things like: rubbing alcohol, nail polisher remover
etc. These things also work but may leave a dull spot on the card
by removing some of the card coating. In some cases, these types
of products can actually remove paint or ink from the card itself.
Some will tell you to use a hairdryer, set on low. This does
loosen up the glue on some stickers but does not work for all
stickers. Plus, you run the risk of softening up the bubble
(potentially misshaping it). Its a bit tricky, but it can be done.
Get the Goo Gone first. You won't be disappointed. If you can't
find it, then try one of the other methods carefully."