Let's preface this post by reminding everyone that I'm not a doctor, or nurse practitioner, or PA. Got that? Use common sense and seek medical help if you're really that badly sunburned.
Okay, got that out of the way. Several years ago, my husband got the worst sunburn over far too much of his body while helping some friends prepare for a lamas ritual. He'd been working without a shirt on and was close to crimson in color, front and back. I hurt just watching him. So, what's a witchy-wife to do? Why, she turns to her herbals of course. This is what a concocted, and, in a move that shows how miserable he really was, this is what Mr. Incredible gave a try. It was almost as good as lavender oil on a regular burn. In short, amazing. (Of course, don't try this if you're allergic to anything in it, okay? Don't want you to make things worse.)
Take equal amounts chamomile, crushed juniper berries, lavender buds, rose petals, and witch hazel to make an infusion. I'd say maybe an 1/8 to a 1/4 cup to about 2 cups water. Pour the liquid into a cool bath and add 1/2 cup finely ground oatmeal. (The contents of a packet of Aveeno's would do nicely.) Soak in the water for about half an hour and finish off with a good lotion.
This didn't just help his pain, but it also reduced the severity of the burn drastically by the next day. I can't swear it will work all the time, since fortunately he's only gotten burnt that badly once.
Another good sunburn relief is straight vinegar applied to the burnt area with a washcloth. You could probably pour some vinegar in a bath and soak in it as well.
Of course, the best bet is to remember to use your sunscreen in the first place, but where's the fun in that?