Store your sweaters in a sealed container. Use a plastic bin with a lid or a cedar chest, either one will do the job. Hang them in the sun to air them out once in awhile.
Dry clean or hand wash your sweaters as often as possible. Washing will kill moths no matter what stage of development they are in. They are attracted to human smells, and removing them frequently will help keep them away.
Make moth-repelling sachets filled with dried herbs. Wrap equal parts mint and rosemary in a piece of cheese cloth and tie it up with a pretty ribbon or a piece of string. You can use lavender this way too.
Store your sweaters with cedar blocks. Moths and other insects are repelled by the smell of fresh cedar. When your blocks lose their scent you can sand them down a little bit to release more natural oils.