Today it's raining, so while I did talk about festival weather already I thought I'd do a few specific rain tips today.
Hopefully you have a waterproof tent or camper van. But just in case of leaks take a roll of black bin liners. Pop all your festival clothes into one, and each day pop your sleeping bag into one too, lay one over anything that won't fit in a bag ... now all your stuff is at least protected from drips, and even if you get wet you'll have a dry change of clothes and a dry sleeping bag.
Just because it's raining doesn't mean it will be cold. Summer festivals can be showery but spells of sun can dry you (and your wet clobber) out very quickly. Don't be disheartened at the first drop of rain.
If you have kids plan a few things that you might do if it rains, meet up with another family? Games in their tent? Or in the rain! Look for events happening inside big tents at the festival (though they will fill quickly if it pours)
Consider taking pack-a-macs (those festival ponchos made of bin bag material are a waste of money IMO, either cut arm holes in a bin bag or take a proper coat). We take Shmangles, a rather fab idea, a sort of cross between a picnic blanket and a cape, with a hood! They can be worn in the wet or the cold, or sat on in the sunshine! And they have a handy carry bag too.
Wear wellies. I tend to wear wellies most days at a festival anyway. They have become such a statement item of footwear. Dry feet are a luxury you can afford. Consider getting some super stylish wellies, so that you'll feel good about wearing them and not look like an out of work farmer.
Don't fear the rain. This too shall pass. It's a festival, have fun, you'll be home in bed in 4 days and miss all this...no really...trust me