MESSENGER will rely on multiple planetary flybys - Earth once, Venus twice and Mercury three times - to "catch" Mercury and begin orbiting the planet. If the spacecraft attempted to fly straight from Earth to Mercury and move into orbit around the planet, it would have required an impractically large amount of onboard fuel (to slow it down) and a much larger launch vehicle.
In its first flyby, MESSENGER will use Earth's gravity to change its trajectory and move in closer to the Sun. An engine burn (known as a Deep Space Maneuver) four months later will alter the orbit slightly and accurately target the spacecraft toward Venus for the next flybys.
The Venus flybys - followed again by a Deep Space Maneuver - resize and rotate MESSENGER's trajectory closer to Mercury's orbit. The three low-altitude Mercury flybys, each followed by a course-correction maneuver, put MESSENGER in position to enter Mercury orbit by firing its main engine and allowing the planet's gravity to "capture" it.
Click here for more detailed information on MESSENGER's flybys.