Faking an illness can get you out of work, school or social obligations that you don't want to perform. You can also fake an illness as a means of getting some attention. The trick is to not use this tactic too often, or you may be branded as a hypochondriac--or, worse yet, you may convince the people around you that you have a horrible, fatal disease.
Don't be specific about your ailment. A vague approach will make your illness more believable. For example, instead of claiming to have a piercing pain in your stomach, you should simply state that your stomach doesn't feel well. If you get too specific in your claims, you may wind up getting diagnosed by the people around you or having medication forced on you. Taking unnecessary medications might actually make you sick.
Don't make any claims you can't back up. Don't tell your family members you have a high fever if one touch to your forehead can prove your temperature is fine. Don't tell your boss you have hives all over your body if there's a chance that she could show up at your door and discover that there are no hives at all. Keep your "illness" an internal phenomenon.
Demonstrate how tired you are. Illness usually makes people feel fatigued, so be sure to convey this to the people around you. Comments like, "Wow, I can't believe how tired I am" or "I just want to sleep this illness off" are a good idea. If given the opportunity, take to bed and get some sleep; this will demonstrate to the people around you that you are indeed tired.
Don't go overboard. People are much more likely to believe your illness if they aren't blinded by your melodramatic performance. Remember that everyone goes through illness once in a while, so people are much more likely to believe you and give you sympathy if you react in a normal and natural way to your supposed sickness.
Act embarrassed that you have fallen ill, and apologize to the people around you for the inconvenience it has caused for everyone.