One of the most common poor sitting postures is Forward Head Posture (FHP), which can lead to serious health problems such as chronic neck pain. To remedy this problem, we propose PreventFHP, an automated system that detects FHP and warns the user of it. Using an inexpensive commercial depth camera, our FHP detection algorithm provides very high detection accuracy. Upon the detection of FHP, PreventFHP delivers warning messages through two sensory modalities, audio and/or haptic. The efficacy of PreventFHP is demonstrated by a user study, which suggests PreventFHP's high potential for improving the sitting posture of computer-intensive workers. The user study also showed that the haptic warning was marginally better than the audio warning in terms of appropriateness as a warning method of FHP.