Remember that the days and weeks immediately following the discovery of the affair are of vital importance, and your actions during this time will greatly determine the speed of your recovery. If your spouse feels supported, loved, respected, and safe discussing his/her feelings during this time, your chances of recovery will be greatly improved. If, on the other hand, your spouse feels alone, ignored, and in the dark, it will be much more difficult to reestablish their trust later.
If your spouse asks you to do something for them to help them recover from the affair (such as read this article, if they have indeed posted it on the fridge), do it right away. Do NOT make them ask twice. Putting off such things only communicates to your spouse that their feelings are unimportant to you, and that you lack the proper remorse for what you've done. Nothing should be more important to you right now than helping your spouse recover from this.
One important factor to keep in mind is that, even though your spouse will ask you to compare them to the other man/woman (was she prettier/sexier, etc), they are also interested in knowing how they are better than the other person, even if they don’t directly ask. In fact, many of these questions are disguised opportunities for you to tell them so. In other words, balance something good about the other person with something you liked better about your spouse.
Pay close attention to your spouse. Your spouse has (probably) never been through this before, and may be too distraught to articulate what s/he needs. It is, in part, your responsibility to try to predict/account for these needs. Whenever possible, avoid making your spouse ask you for things they need from you during this time; doing so puts them in a weak position, and they're already weak enough. Remember: just because they're not bringing it up, doesn't mean it's not on their mind. Be as proactive as possible.
Be on the lookout for seemingly unrelated discussions that may be projections of this issue. Keep in mind that although you may be arguing passionately about who last did the dishes, you may actually be arguing about the affair in some tangential way (i.e. how much time you dedicated to the relationship outside of your marriage). It is sometimes difficult to tell what factors will trigger your spouse's thoughts about the affair - in fact, your spouse may not even realize that they are projecting these issues onto seemingly day-to-day arguments. A good rule of thumb is to assume that any argument in which your spouse seems unduly angry about a seemingly small thing may fall into this category. If this happens, it's unwise to simply back down, as doing so may establish a submissive pattern you will regret later. However, keep in mind that your partner is in an unpredictable and tumultuous emotional state, and be as understanding as you can.
Remember that the paranoia, anger, and distrust that follow the discovery of an affair are natural, and may take a while to fully heal. Your relationship with your spouse during this time is not necessarily an indicator of how it will be from now on; in time, with full honesty and emotional openness, you will (hopefully) begin to recover your relationship's equilibrium. At some point, of course, you may be forced to rethink your stance on whether or not your relationship is terminally damaged; however, avoid making these decisions rashly, as many victim spouses have been known to unexpectedly reach a kind of peace with the affair and move forward just when things seemed at their worst.
It is a mistake to think that just because things are not getting better daily that your relationship will not recover. This is a slow and unpredictable process, and will be different from spouse to spouse. You may find that your relationship improves steadily in the days following the discovery of the affair, or you may notice that some days are worse than others. The important thing is that, on the whole, the process is moving forward; it is only when the process stalls entirely for a prolonged period that you should begin to worry about your spouse's ability to recover.
Be grateful. Your spouse is taking a huge risk in deciding to remain with you after your betrayal. No matter how angry, petty, or unpredictable they get, they have shown a great love for you and, in many cases, a great strength of character in choosing to try to trust you again. Give this decision, and your partner, the deserved respect and gratitude.