1) Buy a curtain that is thick and says mildew resistant. Believe me, it's worth the extra 1.54
2)Do not leave the curtain bunched up after you get out of the shower. This is hard to remember but it makes a big difference. Mildew needs a moist environment to grow. Keeping your curtain bunched means the water in the folds will not dry and mildew has a great breeding ground.
3) Keep a spray bottle with a 50/50 mix of water and ammonia next to your shower. Mist the shower every so often after you leave. This stops mold spores in their tracks. Cheap and very effective.
4)Wipe down your curtain every two weeks. A little dilute bleach doesn't hurt either.
Here's to a mold free shower!