Pray the rosary together, keep yourselves busy, don't spend time alone in the house. Go on dates together, enjoy life.
I've found over the years that the best way to keep oneself from impurity (besides prayer of course) is to have a full agenda. It's true what they say, the devil makes work for idle hands.
In a relationship, prevention is better than cure. But if you do fall, you just have to pick yourself back up and go on.