The pen can be mightier than the sword in a number of different ways. For one thing, the use of swords usually ends in substantial jail time. A pen can tell the truth you want it to, and in conjunction with a log book is pretty convincing evidence. Most of what I'm going to talk about here is rather clandestine. The first thing you want to do, is gather basic information about the people in your immediate circle; this includes your mistress. The information you will need includes: name; date of birth; social security number; their car’s license place number; blood type; where they were born; school(s) they attended; who they dated and when; when they got married; if they were divorced and where; properties they own; memberships they have or had; friends and mutual friends; where they bank; account number (can be gotten off of a check); email accounts; preferences for food, women, sexual fantasies; if they smoke, what brand...etc. You can later augment information by collecting physical elements such as fingerprints (you can get them from a lighter, matchbook, money, or a glass), and maybe a small blood sample. This is a lot of work and is most safely done over a period of time. You are basically creating a dossier on the people around you. In the event they become a threat, their vulnerabilities become very apparent. You can also use some of the information such as fingerprints, to determine if it was your wife who left that vibrator out. You will need to update this collection of information from time to time with notable events which you believe may become useful later on if things turn out: Not Quite So Nice.
If you have violated one of the first cannons of having an affair, which is not to tell anyone, you have become personally, painfully aware that affairs may also be used as blackmail against you. When someone wants something from you, or wants you to perform in a certain way, you will be damn glad you kept that office party picture of ol’ Ted doing the naked bunny hop with the copy repair guy.