This seems like overkill to keep from spilling the contents of your mug, but the SpillNot No-Spill Mug Holder works.
Basically it lets you carry a full cup without spilling a drop. Just put the cup on the base, lift and carry it by the strap and you are good to go. If you practice you can even twirl it around and be safe thanks to centrifugal force. It’s a fun way to enjoy your drink.
Only $13.99 from Thinkgeek.