You also want to be sure the breast shield is large enough for your nipple. Your nipple should not rub the sides of the shield. Medela's web site has great info on this issue HERE (even a video!) and states:
"Many women appear to benefit from a size other than the standard 24 mm breastshield. In one study, many moms were fitted with either a Large or Extra Large breastshield within the first days after birth, and even more moms eventually used these larger sizes in order to pump comfortably and effectively. It is almost impossible to tell which size breastshield is the best fit without watching the nipple movement during pumping.
To determine whether you think you might need a Large or Extra Large breastshield, look at your nipple as it is drawn into the tunnel of the shield during pumping. It should move freely and easily, and should not rub against the sides of the tunnel. If the breastshield fits tightly, your nipple will rub against the sides of the tunnel with each vacuum movement of the pump. After several pumpings, you may notice that the outside of the nipple (rather than the nipple tip) is tender or sore. You may also see a little ring of skin flecks in the tunnel of the breastshield after you pump. While a little circle of milk in the tunnel is normal, a ring of skin flecks probably indicates that the tunnel is too small, and that you would be more comfortable with a larger breastshield. When your nipple moves freely in the tunnel of the breastshield, you will also note a gentle pulling movement in the areola each time the pump cycles. If you do not see any movement in the areola with the pump vacuum, the breastshield is probably too small.
Nipple moves freely and easily Click here to view video
If you have either or both of these symptoms--nipple tenderness around the outside surface of the nipple or problems with breast emptying-- your pumping will probably be improved with a larger size breastshield."