Transits of Venus across the disk of the Sun are among the rarest of planetary alignments. Indeed, only six such events have occurred since the invention of the telescope (1631, 1639, 1761, 1769, 1874 and 1882). They always come in pairs, 8 years apart, every 120 years. The next will happen in 2012 and then not again till 2117.
Here’s a guide on how to see next year’s, courtesy of Flickety on wikihow.com.