An effective goo remover is peanut butter (aka peanut paste). In
the absence of peanut butter, use WD-40. Both leave a smudge of
oil, but that's easily cleaned up with a drop of detergent on a
damp tissue.
On a metal or glass surface, tea tree oil and mineral turps are
both good at removing sticker goo, but these can attack paint
and plastics so don't use on such surfaces unless you are
willing to accept some surface spoilage.
Peanut butter? Heh! Now that will give me an excuse to keep a
jar of peanut butter in the house.
Must try the peanut butter because although I haven't tried the
tea tree oil you suggest for removing sticker glue, I have tried
eucalyptus oil and it works rather well. Presumably eucalyptus
oil is quite similar to tea tree oil in this respect.
Now here's another: if you are a sweaty sort of person then I
find that skin oil (say, from the brow) works remarkably well!
