You have a lot of work to do. You're on a deadline. You're stressing out because people are constantly dropping by your desk saying hi, how you're doing, because you're in a computer lab. You need to block everybody out. What do you do? Slap on some headphones and tape this sign onto your back. That should do the trick. You can work without any interruption now.
The hilarious, real-life do-not-disturb message gets all the niceties out of the way and explains your situation. Anyone who sees that sign and still wants to bother you is a complete jerk. Anyone who wants you to stop working doesn't respect your time! Though the person in the picture looks to be an art student, this message should work for anything -- at work, at the library, at the salon, etc.