Lie on the floor and place a tennis ball under each glute just below your pelvis (where your pockets would be).
Then drop both knees out to the side whilst placing the soles of the feet together (think frog!).
Gently roll around to see if you can find any tender spots. When you do ease into it gently and don't forget to breathe! You should feel the discomfort reduce. If it increases then move off and look again.
Stay on an area until it reduces or for around 60 seconds (whichever comes first). Find a couple of different 'spots' to work on.
Massaging these areas using the tennis balls little and often is best. You can also do the same thing with a single tennis ball.