As you get older, you often lose muscle mass in your bottom. I found that if you use a variety of exercises including dumbbells, stationary holding positions and using the squat bar.
Like other exercises it is important to have good form, control and using the correct amount of weight.
I have personally found buttock exercises a bit embarrassing due to the fact in order to achieve good form, one must stick ones bottom and boobs lol!! Sometimes awkward when the gym is full of the opposite sex but necessary to achieve great results.