This muscle tension may be caused by:

Inadequate rest
Poor posture
Emotional or mental stress, including depression
Anxiety
Fatigue
Hunger
Overexertion

The most common sources of stress include family, social relationships, friends, work, and school. Examples of stressors include:

Having problems at home/difficult family life
Having a new child
Having no close friends
Returning to school or training; preparing for tests or exams
Going on a vacation
Starting a new job
Losing a job
Being overweight
Deadlines at work
Competing in sports or other activities
Being a perfectionist
Not getting enough sleep
Being over-extended (involved in too many activities/organizations)

If it's the muscles, work on the first list to ease, if it's social/environmental, work on the second!

From: http://www.medicinenet.com/tension_h...ticle.htm#tocc