You don't even have to wait until your contract expires if you don't want to. She can perform an AOL(assumption of liability) to take over HER line and leave you with an individual account.

In the future, if you do not want to be on contract you would simply pay full retail for any phones you purchase instead of the greatly discounted prices you would pay when signing a contract. For example the "contract" price of a 16 GB iPhone 5S would be $199.99 vs the "full retail" price of $649.99. By paying that extra $450 for the phone, you DO NOT have to sign a contract.