Take a few deep breaths to clear your mind. If practicing your guitar feels like a chore or if you are apprehensive about learning a new piece of music, chances are you will be tense. If necessary, listen to a few tunes that inspire you or remind you why you wanted to pick up the instrument to begin with. You should see the activity as recreation, not a task to complete.
Tense your muscles for a few seconds, then relax and massage them. You don't have to be as meticulous as a masseuse, but rubbing out a little tension can go a long way. You should ideally be relaxed all over, not just with the muscles that you directly use to play the guitar.
Stretch your fingers, as an athlete would stretch his body before an event. Playing scales or a few bars from a demanding song are a good way to do this, as long as you do it slowly. If you jump right into the thick of things, you risk cramping up or, at the very least, harboring unnecessary tension.
Play a chord with as much pressure as you can, then repeat it with as little pressure as possible. This is a good way to teach yourself that you don't need to play as hard as you may think you have to.
Exercise economy of motion. Though it may look cool to move your hands all over the fretboard, it can become tiring after a while. Stringed instruments offer several ways to play the same note; play the ones that require the least movement. For example, when an octave is needed, play it two strings up, two frets away. This is much easier than traveling 12 frets up to achieve the same note on the same string.