Don't plan on a ready market for your sublet as there are generally a limited number of students who look to sublet during the academic year. For sublets during the summer -- June, July, and August -- be prepared to get approximately half your regular monthly rent due to the abundance of available units.
The best places to advertise a sublet are in the local newspapers, through word of mouth, and with an off-campus listing through the Cornell Apartment Listing Service. Remember, don't ask for an unreasonable amount of rent, and always be ready to negotiate.
Assuming you find someone to take over the lease, it is best to have the landlord void your lease and sign another one with the new tenant. If this can be done, you are completely free of any obligation to the landlord. If the landlord is not willing to do this, you remain legally responsible for fulfillment of the terms of the lease, even though someone else may be living in the premises.
If you sublet, you should have a written sublease agreement between you and the new tenant. A sublease agreement should be identical to your lease with the landlord and hold the new tenant to the same obligations binding you. An apartment or dwelling can be sublet unless the lease specifically forbids it.