1) Get therapy with a reputable expert in infidelity and discover the factors (not excuses) that led to the affair so that alternative healthier solutions can be practiced.
2) Look for behavioral change (depending on whatever was discovered as to what was behind the secretive actions) in the relationship that lets you know that something different is truly happening and that there is reason to take a chance on your partner again.
3) Whenever there is a “risky” situation such as going to a work meeting where the “other person” is attending, the one who betrayed needs to go out of his or her way to reassure the partner before, during and after the meeting (or whatever the event or location is).
4) Spend consistent quality time with each other and both people should use the relationship as a crucible for increased self-expression.
5) The person who had been betrayed has to on some level be willing to get hurt again; that is not to say that you plan on it and it is not being masochistic. It just means that at some point when there is enough reason to take a smart risk, go in all the way and be willing to be vulnerable again. Holding back long term poses risks as well. Accept the inherent risk that relationships pose. In order to feel loved, we need to be open to hurt. Of course the person who previously cheated needs to be appreciative and sensitive to the risk the partner is taking and of course needs to continue being honest and transparent.