3) Write a song. The international song market is absolutely dying for a feel-bad song written from the perspective of a penitent cheater. The only problem is that this song could make you famous and that's when groupies and/or John Mayer start lurking around…try not cheating then.
2) Be there. Seriously, nothing says, "there is no way I will ever cheat on you again" like always being around someone. When you have to be away, like at work or whatever, check in regularly… every 15 minutes should do it. Feel free to forego sleep too. Your partner will feel instantly reassured if they wake up with you staring intently at them.
1) Do stuff you don't like. Eating gross (but not dangerous) food is a solid start. But nothing says suffering like spending hours and hours doing something you loath. And the less happy you look about being there, the more your spouse knows how sorry you are
*Note: Please keep in mind that this list is satire (though Taylor's punishment was not). If you and your spouse are having issues, you should probably seek help, as infidelity may be symptom of a much larger issue. And don't cut any body parts off.