A long time ago I set myself some ground rules.
1. Marriage first - don't get emotionally attached to anyone else. If you feel you are going too far over the edge cut the affair off.
2. She needs to loose as much as you - Try and find someone who is in the same boat (married, kids, house, career etc). This is almost an insurance policy that guarantees that discretion will be kept by both people.
3. Cover your tracks - This is what gets most people in trouble. Create a new secret email account make up something random. Make the password hard too, use a long password for everything, with a combination of letters, numbers and special characters, for example '1DietSoda4$-'. Use Google chrome in porn mode! How many people do you think were caught because their spouse looked up their Internet history, porn mode pretty much eliminates that, and don't forget to log out every time you walk away anyway. Don't use AIM, it stores conversations, and even if you log out there is a chance that a conversation will pop back up. If you do feel the need to save pictures, put them in a random folder wherever you store your porn, that way it looks like downloaded porn.
4. Don't use your credit card - You will get a credit card bill in the mail, pay in cash, some hotels require a credit card, do your best not to give it to them, go to a different hotel. Some people also use pre-paid credit cards, I have no experience with them, but it seems like a good idea.
5. Don't use your cell - Get a cheap disposable pay as you go cell. Spend the $25, if it is ever found, the best excuse is that you found it, then get rid of it. If you text or call on your regular phone the bill will come in the mail, the texts will be seen, the pictures will be seen, the numbers will be called.
6. Get a PO box - If you need to get a bill for anything, get a PO box, it's cheap, like $35/yr, have them sent there.
7. Feel her out first - every first meeting should be somewhere public, coffee shop or a book store, something along those lines. It gives you an out if you feel uncomfortable or you don't like her.
8. Don't talk about it - vent somewhere anonymous, go to a catholic church and into the confession booth. Priests don't talk just be careful because if they see you in the supermarket they will suggest you go to marriage counseling, true story.
9. Pictures last - Don't take any pictures of you and her. They have a habit of surfacing later. Words are easy to dispute, pictures aren't. If in correspondence with a potential surrogate wife, you decide to exchange pictures, make sure the picture doesn't include your face, or any distinguishable marks on your body. Don't try and crop pictures either, the data is still there if you know where to look, take the picture missing your head, put some tape over any markings, make the picture physically unidentifiable as you. With the ink on my body any picture I would send looks like almost like a black blob. Plus I cover a few added areas just to be safe and make it look like it's not me.
10. Be safe - There is nothing worse than your wife telling you she has VD, except she hasn't been sleeping around, so where did you get it from? I always use protection, and luckily this has never happened to me, but the danger is there.
11. Be transparent with your alibi - Your alibi is the key. I do my best to tell the truth, minus a few details. I'll say something like "I'm going out for a ride on my bike, be back in a few hours". I just didn't say where I went or with who. Also make sure your details are straight, and you know what you did, don't give too many details unless you have too, the more intricate the story is, the more you will make a mistake or forget something.
12. Don't be too anxious - If you take every opportunity to meet with the other woman you will be pushing your luck. If you are going out for a long time, come home early once in a while, if your wife wants you to stay in, don't blow her off, stay in.
13. Clean yourself - Take a shower, take two, use soap. Sex smells, it smells like sex. It is a distinct smell, and if you didn't have sex with your spouse that day and you smell like sex, the question is why, and it isn't a question that you want to answer. I take a shower after sex every time. If I'm at home it might be the next morning but I do, sometimes I even take a shower just because, on an average day I shower three or four times a day.
14. Plan B - I keep a gift for my wife in the garage. If my location is ever questioned I will present that present and turn the tables. "I was out buying you a nice surprise gift! How dare you question where I was!". It's a great out, it gives you a story, and it gives you a place, and it makes her not wonder were you were.
The biggest thing that you really need to know is that discretion is the key, keep your secrets secret, keep the secrets of others secret, know their intent, and cover your back before you walk out the door.