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The easiest way to get them off when they're too tight is to take it by the middle (don't squeeze the valve too hard or it will snap the area where the membrane goes) and jiggle it back and forth until you feel it come loose...then you can twist it off.

Use a nipple brush. You can't just soak them or rinse. After using the nipple brush sterilize it once all the mold is out. And, don't store them in a bag or anything still wet or they will get moldy, especially if you have a lot of humidity in your house.

I definitely agree with Liamsmom but, one of the websites posted was an error...here is the correct one.  http://www.ncsl.org/programs/health/breast50.htm And in the LLLI website go to the very bottom and click that link and select your state.  Good Luck!