Plane, car, or bus are your basic options.
By plane
Saltillo's International Airport IATA|SLW is located on the north side of the city, about 10 minutes from downtown. It is served by a regional carriers to Monterrey and Mexico City. There is one daily flight from the U.S.: a Continental Express flight from Houston, however, convenient flight schedules are available to nearby Monterrey.
By car
Saltillo is one hour west of Monterrey via Mexico Highway 40.
By bus
Many bus lines serve Saltillo, including Omnibus de Mexico, Transportes del Norte, Turistar, Futura Estrella Blanca and Coahuilenses Grupo Senda. Trans-border buses to the U.S. are also available from Saltillo.
From Monterrey either the central bus station or the airport there are hourly buses going to Saltillo, run by Coahuilenses. Also, these make two stops inside the city before arriving to the bus terminal. For most hotels, you'll want to drop off at "Hospital del Niño" first stop, in the northern part of the city, and take a taxi there. In this same spot, across the avenue, you can take a bus back directly to the airport.