The moisture generally comes from your mouth. If you've recently eaten (especially sweets) or had a sweet drink (or alcohol), your saliva glands will be working overtime, and more of that will be blown into the horn. Often, it will come out around the keys of the left hand. First, try not to eat and drink just before playing. Secondly, you can open up the left-hand palm keys and try to blow at the openings to try to get some of the moisture out. Also, be sure to swab the horn after each use, to be sure you are not leaving moisture inside the horn during non-playing times. That can degrade the pads quickly.