Lunges help a lot, especially to get a good "Jeans" Bum. BUT to make sure you really work had you need to add a step too. Get into a normal lunge position, but instead of your front foot being flat on the floor put it on a step. Make sure you are on the toes of your back foot, do not let your heel down. When lunging ensure you lunge straight down, making sure your front knee does not move over your front foot (keeping 90 degrees). You get a more intense lower lunge, it really hurts, burns like hell, but your bum will hopefully show your efforts.
Hope I have explained that alright.