So there you are in front of the Taj Mahal or some other tourist site and you want to take a photograph of it. Unfortunately other people also like these sites and wander across the frame of the photo as you are setting up. Some will notice you and be considerate enough to wait for you to finish or walk behind you but most will be oblivious that they are ruining your shot.
OmerRAnderson suggests a simple way to use technology to eliminate all transient intrusions into your photographs. It seems ingenious to me and I pass it along even though I am not a photographer and have no idea if this actually works or not. Some commenters raise problems and suggest ways to improve it.
Of course, this does not solve the problem of people who hang around for a long time. With such people, you may have to revert to the old-fashioned way and politely ask them to move.