More information is contained in the Clarinet Handbook.
Every musician deals with tension, whether it's mental tension that arises when performing or physical tension that changes the sound, inhibits fast, fluid fingers, or otherwise impacts music-making. An awareness of your personal tension habits can help you become a better musician (and alleviate pain).
To assist in this, I suggest filling out a Personal Tension Inventory detailing your tension while you play when seated AND when standing. After this, use the Tension Diary to work to reduce tension in problem areas (one per week). The attached readings can help you in this process; of course, there are many resources available, and I strongly encourage you to do additional research. Lastly, the Wellness Resources on our Links page may also prove helpful.