For about $4, you can buy a vinyl shower curtain that resists mildew and you won't have to do all the extra things that have been suggested; however, it is always a good idea to close the liner when not in use. You can get the mildew-resistant liner at Walmart, Target, K-Mart and similar stores. I have even seen them at Dollar Tree.
Too humid in your bathroom. Run the fan if you have one when you shower. Always keep curtain closed after showering so it can dry faster. Invest in a fabric liner that doesn't mold quickly, and wash it every two weeks.
As you have already been told, there is to much humidity, my friend had the same problem...and because mold can be a health hazard...she bought a dehumidifier and sat it by the window and let the water fall outside. It works very well.
Humidity and wiping it aside, when you're done showering. Open the curtain up, don't leave it bunched up on one side of the tub. Bunching it up just causes the water to stay in it which will cause mold much quicker.