In addition to the suggestions above you might try the Flashlight test (this will not work so well if your humi has any glass): put a lit flashlight in your humi, and go into a darkend room and see if you can see any light. If you can see light you have a leak and the light will give you an idea where the leak is so that you can fix it.
As has been suggested, puttling some weather stripping around the lid may help.
Finally, you don't mention this, but if you humidor has any glass then the seam where the glass meets the wood is likely the source of a leak. If this is the case, get some clear silicone sealer (caulk) and seal the seams around the glass. Shilala has posted a
nice guide that explains how to do this right, your you can do it the easy way without removing the glass. Your choice.
Make sure to give your humidor plenty of time to air out after applying an sealant.
Good luck!