Do your research. Know which rental companies will rent to drivers under the age of 25 and compare their fees associated with age. Depending upon your location, you could try Advantage, Avis, Budget, Dollar, Enterprise, Hertz, National, Thrifty or Rent-A-Wreck.
Know if you have any options that will allow you to rent a car for a reduced fee. If renting a car for business, check to see if your company has a corporate account with a rental car agency.
Be prepared. Have your credit card information, drivers license and insurance information readily available. Be prepared to pay a much higher amount than someone over the age of 25.
Make your rental arrangements over the phone rather than online. This will allow you to speak directly to a representative about your situation, enabling you to get the best deal possible.