The image below details Juno's trajectory. Each tick on the path represents 30 days. At first, it may seem odd, but take a close look.
Juno will be travel away from Earth for a year, passing Mars, and slowing down until it reaches its max distance. Then it will return to us, using a combination of the Sun's and Earth's gravity to accelerate, doing a flyby in October of 2013. Notice how much faster it is moving at that point.
From there it is a nearly 23 month journey to meet up with Jupiter, using the Sun's gravity to curve it as it goes, meeting Jupiter in July 2016.
Sometimes you need to plan ahead