Confidence is the most important part of telling a believable lie. Without it you may babble, fidget and give yourself away. If you have decided to tell a lie, you have already justified in your mind the necessity for it. Therefore there’s no reason why you shouldn’t be able to pass the lie off calmly and confidently. Here are a few things to remember that will help to boost your confidence:

You have nothing to feel guilty about. Remember that you are doing nothing wrong. You are the underdog in this situation, the person who must ask for a day off. You need a day off but you are not able to get one without a lie, and that is not your fault.

The person you are telling the lie to is beneath you. If your boss cared about you, you’d be able to reason with him. You’d love to tell the truth! But in this inhumane corporate rat race of a world telling the truth would risk your livelihood and future. See him for who he is: someone who’d fire you and not feel bad about it.

Keep your eye on the prize. Remember why you need the day off and do what you have to do to get it. Trick your brain into thinking about life like you are a player in a game. Go in there, get the task done, and get out. It will only take a few minutes and you can turn back into a nervous human being afterwards.
(Or perhaps you need a doctor’s note? See here)

Once you have shed any unnecessary guilt, fear and anxiety you are ready to plan your performance. You must come up with a believable story that you are able to speak about naturally. Here are a few tips that will help your story sound genuine:

Believe what you are saying. To tell a believable lie, you must believe it yourself. Prepare yourself like an actor by mentally going through the story in your head and imagining that it really happened. Close your eyes and feel the food poisoning in your belly.

Ad enough detail to sound believable. Make sure you ad a couple of irrelevant details into your story so that you sound like you are recounting an honest memory. An example of a detail could be: “So I’m fumbling for quarters to get my three-year-old one of those bouncy balls and then—oh man a sharp pain in my stomach.”

Don’t say too much. Although details are important for credibility, less is more. If your boss doesn’t ask, don’t tell him. If you are telling the truth there’s no reason to blurt things out and babble. Talking too much will make you sound nervous and you will end up adding more to your story that you will have to remember later on.

As long as you keep these things in mind there’s no reason to let your nerves get the best of you. You are not lying to cheat anybody, or to hurt anybody. You are lying to take back a basic human right: a day off!