My H and I use our version of the Marriage Builders program, and we do feel we are specifically affair proofing our marriage as much as is possible. It feels great, actually!
Their plan is simple but very complex to execute. It was so worth it to us, however.
They start by teaching you the basic premise that to have a lasting monogamous happy marriage, you need to stay romantically in love. So they give you a bunch of new ideas to work on, where you learn how and why a couple become (and stay) romantically in love. You also learn why couples fall OUT of romantic love. Then you learn what behaviors you both should do (and not do) that helps keep you romantically in love with your spouse. Mostly this has to do with fulfilling each other's emotional needs, but there is a lot more to it. I found the reading material somewhat hard to get through, but once I understood the concepts I started really flashing through it all easily.
Then you surround your marriage with strong boundaries that basically say "no other may fulfill my emotional needs".
Some of the behaviors of enforcing these boundaries are things like:
No friends of OS.
No GNO/BNO.
No hidden emails, phone numbers, other accounts.
No same sex friendships with toxic friends.
No telling anyone of the OS about marriage problems.
Those are just some examples of tight boundary enforcement. My husband and I are not 100% tight on the OS friendships and GNO/BNO. We both do have some totally safe OS friends, and we have totally safe same sex friends for GNO/BNO. But other than that, we are pretty strict in these areas.
We do not pick up "new" friends of either sex, here and there. We have a pretty tight grip on outside influence of any kind. We each have several long term friends and some acquaintances, but we aren't looking to add more friends. We do not go out and drink in bars with our friends of either sex (without each other), as it is simply not a good healthy marriage activity, we feel.
We remain deliberately focused on each other, and refuse to even acknolwedge advances of others. We treat each other with the highest regard and respect.
Because we learned the Marriage Builders concepts and plans together, we still talk about how we are deliberately protecting our boundaries, so there are no questions about it. We even "check in" once in awhile with each other by one of us looking sternly into the others' eyes and saying "you know I'm gonna check up on you if I feel I need to, right?" and the other will reply "of course, and the same for you, but I invite you to check out anything I do or say at all because I am all about you and have literally nothing to hide". Then we smooch and remark about how good it feels to know we are each doing our part to keep those boundaries shut tight.
It makes me swoon, actually...the individual acts of self-discipline involved in keeping these tight boundaries that we both have to execute really do keep the romance burning.