Leaning in closer to your computer as the workday drags on is an occupational hazard, and it's one that can play havoc with your lower back. Save money on massages (though lose the slightly-shameful feeling of paying someone to touch you) with the LED USB Computer Posture Correcting Alert Device. That's a very long name for a pretty simple gadget: it perches, webcam-like, on your PC, flashing and beeping if you lean in too close.
A blue light on the device tells you you're sitting the correct distance from the display, but if you get closer than 35cm that changes to a flashing red light and, if you haven't disabled it, a loud chiming. Power is supplied by the USB port but there's no other use for it; don't expect data logging of your posture or anything so complex.
Looking at the design, the Posture Corrector seems more intended for desktop PC than laptop use; you may have trouble perching it on top of your MacBook, for instance. Still, if your kids are perpetually leaving eye-juice smears on your CRT, $21.69 might not be too much to spend.