First Effort in "Right Effort"

The first effort states **to prevent the arising of unarisen unwholesome states** How is this done? How can we prevent something beforehand? By avoiding situations? By trying to be in a rational and/or 'dhammic' frame of mind? If there is contact with an object, either ignorance or wisdom arises subsequently in response, and AFTER THAT, we can use sati-sampajañña to get rid of the unwholesome state (2nd factor of right effort). But how is the first factor to be practised?