[r9etb]

It's a matter of "orbit-fu." They don't have a lot of rocket power, so they're using gravity assists to bleed off the energy of the transfer trajectory. From the article:

MESSENGER, which stands for Mercury Surface, Space Environment, Geochemistry and Ranging, was launched in 2004. The probe has meandered through the inner solar system on the way to Mercury, already having flown past Earth once and Venus twice.

A more detailed view from here:

They have four flybys between now and 2011.

[rednesss]

And unlike Mars, Mercury doesn’t have an atmosphere so there will be no aerobraking possible.