Para in front of an infinitive means "in order to." Quiero visitar España para mejorar mi español. I want to visit Spain (in order) to improve my Spanish.

You're right, I think the most common preposition before an infinitive could be "para" because as Spagnola said it means "in order to" most of the times. Actually you can use another prepositions before an infinitive: "Cogí un resfriado por salir a la calle mientras llovía" (I caught a cold for going out while it was raining) "Tras cantar la última canción, él público se puso en pie" (After singing the last song, the public stood up) "¿Qué piensas sobre (lo de) hacer el proyecto?" (What do you think about doing the project) These are just examples, another prepositions could be also used in another situations before an infinitive. I hope it helps ^_^