Other than a good rain gear and a tent with fly and and vestibule, The 3 T's have have all of our rainy camping trip bareable, even enjoyable: Trenching, Tarps and Tea. Trenching isn't great for the earth so try to choose your sites with a downpour in mind. Keep you fire, cooking and eatings areas dry by roping tarps. Keep water boiling for hot drinks like tea, coffee and hot chocolate. These will go a long way to keeping morale up.