Well, if you're going to do it then you're going to do it regardless, may as well give you some tips to help ensure he doesn't find out.
- Be careful of your correspondence. If you're using your cell phone to talk to the other guy, clear the history, erase the text messages, etc. If you call him on your normal phone, hit a bunch of random numbers afterward so his number's not on your redial. If he calls you, erase it off your caller ID.
- Be careful who you tell. You may think it's okay to tell one or more of your friends about it, who you're super sure would never betray you. However, it only takes one time of pissing them off or them feeling bad for your boyfriend, and the secret's out. This is especially true for your family members, who are even more likely to speak to your boyfriend and thus more likely to even just accidentally let something slip. Whatever your cover story is for your boyfriend, use the same one on your friends and family. You need to be consistent to avoid suspicion.
- Use cover stories that can't be verified. If you are gone for a while, and he wants to know why, you say you went for a drive or a walk. You can stop at a store and pick something up on the way back as proof you went to the store. What you don't want to do is to say you were at your friend's house or visiting your mother or something, where he can verify by calling said person. You also don't want to pretend you're going to a class or place of business (like a doctor's appointment), because it's also possible to verify this.
- Keep his schedule in mind. If your boyfriend is working, going to school, or otherwise busy for an extended period of time, that's a perfect time for you to be cheating on him. You don't need to explain your whereabouts, so it'll be easier on you as well. You also don't want to be seen in public somewhere with the other guy when it's possible your boyfriend may be there himself.
- Never bring the affair home. This is the biggest mistake people can make. You can never be 100% sure of his schedule, even if he's going to the airport to fly to the other side of the world, he may miss his flight or come back early. Never, ever, bring the other man back home. It's never worth the risk.
- Always have a backup plan. For example, purchase a watch or a tie for him from somewhere, pay for it in cash, and throw the receipt away. Then, hide it in your truck or under your car seat. Keep it there forever. If you ever get home and he demands to know where you were, you can break down into tears and say you were just out buying him a present because you love him so much, and you can't believe he doesn't trust you. Then you can tell him where the present is in your car as proof, and he'll feel terrible. The reason you throw the receipt away is so he can't check the date you bought it.
- Always act like you're completely loyal. If he ever doubts your faithfulness, get upset and offended, and say you can't believe he wouldn't trust you. Say you've never given him any reason not to trust you, even if you have. Ignore any facts or rationalizations he presents and respond by turning it back on him, like he's the one doing something wrong by suggesting it. Immediately justify anything he says is suspicious behavior, then make him feel bad for thinking it was suspicious.
- In a worst case scenario, introduce the other guy to your boyfriend as "a good friend of mine." It's better to avoid this if possible, as the chances of your boyfriend becoming suspicious will increase dramatically. If he gets upset over this friendship, make a point of making it seem perfectly acceptable. Good examples would be "What, I'm not allowed to have any guy friends now?!" and "Are you going to decide what I can wear next too?!" and "Wow, so you really don't trust me?!"
- Look for an opportunity to make the break-up his fault. If you want to end the relationship, just wait for him to do something wrong, even if it's something insignificant. Blow it way out of proportion, make it into a huge deal, then point out his massive failure in a format similar to "If you can't even ... then maybe we shouldn't be together." If you can make it seem like it's his fault, then you can get out of the relationship without breaking his heart. At the very least, he'll blame himself for the heartbreak.