I have a digital compass that has 3 suction cups that are used to
attach it to my windshield. However, the compass falls off my
windshield every few days. I've tried licking the suction cups and
then putting it on the windshield, but it just keeps falling off. Is
there a stickum I can use to keep it attached? I suppose I could just
use glue or something, but I'd prefer not to use something that's
completely permanent.

Howdy mxnmatch!

You should use clear caulk for something like this, such as
the following from DAP.
http://www.dap.com/retail/retail_detail.cfm?subcatid=2&catid=1&prodhdrid=7

"DAP® Kwik Seal® Tub & Tile Adhesive Caulk Bonds like a glue
and seals like a caulk."

The above costs around $3.00 at hardware stores like True
Value, and what you have left can be used for all sorts of
things around the house.

It sets up quickly (mositen two of the suction cups, and
apply the caulk on the third cup at first) and at a future
point you can use a straight razor or utility knife blade
to remove the caulk from your windshield with no problem.

DAP also has a product that can be used for caulk removal.
http://www.dap.com/retail/retail_detail.cfm?subcatid=3&catid=1&prodhdrid=12

DAP® CAULK-BE-GONE® Caulk Remover

"Specially formulated gel softens latex caulk for easy
removal. Fast-acting formula, works in as little as two
hours. Fresh scent. Will not harm surfaces. Use for
indoor and outdoor applications."

Search Strategy:

Searched Google with the keywords: clear caulk
://www.google.com/search?&q=clear+caulk

If you need any clarification, feel free to ask!

Looking Forward, denco-ga

I throughly endorse denco's suggestion of clear caulk for this sort of
problem. Back when I used to wear contact lenses, I had difficulty in
keeping a tiny magnifying mirror in place on my main bathroom mirror.
Like your compass, the suction cups kept loosening. I put a dab of
clear caulk on it, and the problem was solved. Clear silicone caulk
comes off very easily if you want to remove it, leaving no residue.