This fact got me thinking about how people weasel out of going work, and in a happy coincidence, I found some advice from Lifehacker on how to create a more believable "sick" day.
One key is timing. Pick a random Tuesday or Wednesday for your fake illness and it'll be more likely to fly than, say, the Tuesday after a long weekend. It's obvious, of course, that Mondays and Friday are prime days to tweak your boss's suspicions, so save those for when you're actually sick.
Call, don't e-mail. E-mails can seem pretty fake, and there's the risk that your boss might decide that if you're online, you can just work from home. If you can, call his voicemail directly to avoid uncomfortable questions.
Don't get caught up in the details. Too much description can sound fishy. Just say you have a migraine or food poisoining. No one wants to know what the contents of your stomach are currently doing, so you'll be spared the third degree.
Pick a good malady. Speaking of food poisoning, that's a great non-specific ailment that typically resolves in a day. Claiming chicken pox, on the other hand, would require extra days off, a doctor's note on return, and possibly some makeup artistry skills.
Don't be stupid about your freedom. Posting your exploits on Facebook or showing up with a tan the next day are rookie moves. 'Nuff said.