Nothings easier then dipping a PG tips in a mug of boiling water, but unfortunatly thats not always the case while wild camping as i've found out;
Pine Needle Tea - Get a few sprigs of Pine Needles and slice in half with your knife or a rock, then put in your mug of boiling hot water and crush as much as you can with a spoon or a twig? don't boil the tea anymore or else you will start boiling off vital Vitamin C. Pine Needle's have the same amount of vitamin C as half a douzen lemons ;D Pine Needle tea a day keeps the colds away!
Nettle Tea - Get a few baby nettle leaves, be careful though these will sting you! perhaps wear gloves or pull them off with some sticks. then put them in some boiling hot water and mash them up.
Dandelion Tea - The edible parts are the roots, leaves, flower head and petals, don't eat the stems as they have the milky liquid inside them and this will upset your stomach even more so then any other parts of the plant. baby dandelions are the best as they aren't as bitter. Just be sure to make sure you don't get any sickness from this tea as you'll become even more dehydrated from being sick.
Mint Tea - I've taken a small baggie of Mint leaves before while wildcamping and made a really nice minty water brew ;P can't beat it especially with a nice warm fire and some soup.