Batiste dry shampoo is a life-saver! I'm never without a bottle. I use the 'Medium Brunette' dry shampoo, which is perfect for brunettes as it's tinted, so doesn't leave your hair with a horrible grey tinge. It usually costs £3.99 but at the moment it's on special offer in Superdrug so you can pick up a bottle for just £1.95!
I've stocked up :)