IMPORTANT: Make sure you know what poison oak, ivy, and sumac look like and avoid them! Some fuzzy leaves, like mullein, can also cause a rash.
Wash your hands often**
This no-brainer only gets tricky if you get caught without soap or hand sanitizer. In that case, you can wash your hands with saponin-bearing plants. Yucca roots and leaves; sweet pepper bush flowers; bouncing bet (aka common soapwort) leaves and roots; and clematis leaves and flowers can be crushed in a little water to produce suds. Don't just try strange plants, though. Make the wrong choice and your skin can absorb toxins and produce a rash, at the very least.
Keep your teeth and gums clean and healthy**
Make a toothbrush from a hardwood stick like oak or hickory by cutting off a live twig and crushing one end to make it fibrous. You can also improvise dental floss from a variety of non-toxic cordage fibers, and toothpicks from dull thorns and wood splinters. Just be careful not to injure your gums. If you do poke your gums and they feel infected later, rinse your mouth with acorn water. Acorns and their shells can be boiled to release tannic acid. Swish this water in your mouth for a few minutes at a time a few times a day, until the irritation subsides.