Usually, "hasta luego" (until later). This is different from "adiós" (good-bye), which is what you say without any expectation that you will be seeing someone later, for whatever reason.
I found a great postcard once, by Ann Taintor, with a smiling señorita and the caption "her lips said 'adios'...but her eyes said 'hasta luego.'" : )
![[Ann Taintor card: Her lips said 'adios'...but her eyes said 'hasta luego.']](http://www.spanishpronto.com/spanishpronto/her_lips_said_adios.jpg)
Unfortunately for Spanish teachers everywhere, at last check, this did not seem to be available any longer. If you find out different, let me know!