Make sure that the ball is blown up enough that you knees are lower than your hips, your pelvis will tilt forward a little. Small bouces up and down, side to side and forward and back. Circles and figures of 8 keeping your feet firmly on the ground all the time. Sit with your back straight and imagine you are being pulled up from a single piece of hair on the crown (back /top) of your head, this will put you in the best position and help the baby get settle down. Remember to breath try all through, try and make the out breath longer than the in breath, close your eyes and relax. You can also kneal (sp) on the floor and rest your top half on the ball, rocking froward and back. If over 30 weeks try putting you knee up on each side (one at a time) stretching out the pelvis (don't if you have SPD or pelvis pain)
My fav is to put the ball on the wall and in the small of your back and just bounce off the wall a little. It really feels good. Make sure it is in the small of your back. You can also stand and lean on the ball, dipping the small of your back but I don't get on with this one - feel a bit too silly I think.
Good Luck I love my ball