In the expanded iTunes window (not the tiny one), select the song, start playing, drag the time/pointer thumb thingy in the progress bar-like display almost all the way to the end (right) and let it play the last couple of seconds of the track. When the track is done iTunes increments the playcount.
[Editor's note: This trick is quite useful if you're using a smart list such as the one discussed in the "Most ignored" playlist hint.]

