Take stock of your clothing. If you're wearing cotton, it's going to absorb moisture and will take forever to dry. Better to strip down to your base layers if possible. If you're wearing synthetic fabrics, they'll dry much faster. Try to find some shelter. If there's a tree nearby, stand under it. If not, huddle under your raincoat or tarp. Stay calm and keep moving. It's easy to get cold when you're wet, but if you keep moving, your body will generate heat and you'll stay warmer.