If you don't want to commit to the higher monthly costs of a 15-year mortgage, you can reduce the amount of time you need to pay a 30-year mortgage by paying more than required.
You can do this by paying extra each month or through occasional lump sum payments. A popular strategy is making payments biweekly rather than monthly, which results in one extra payment each year. Any extra payment will also lower your total interest costs, since they should go toward reducing your loan principal.
Most lenders will allow you to make extra payments without a prepayment penalty, but be sure to ask. Also confirm that your additional payments are being credited toward your principal loan amount, not future interest payments.