The British Chiropractic Association (BCA) has advised that users take a 'Wii Warm up'  before playing the Wii.  From the BCA site:

Tim Hutchful from the British Chiropractic Association comments: “As chiropractors we are encouraged that this new technology allows users to be so active especially at this time of year when the tendency is to do very little except eat and watch TV! We want people to enjoy these games, but enjoy them safely. Gamers should treat a ‘Wii session’ like any form of exercise and warm-up adequately beforehand to prevent stiffness and possible injuries.”

The BCA has released a paper called 'Wii Warm up' which is similar to the advice WorkoutWithWii posted when the Wii first came out.  So now you have two independent sources recommending a good stretch before a Wii session.  From the BCA paper:

The BCA has developed a ‘Wii Warm Up’ to help users retain nimble fingers but
avoid over exertion:

1) Shoulder Shrug – Slowly shrug your shoulders towards your ears. Hold for 2/3
seconds, and then relax. Repeat 3 times. Because it is easier to relax a muscle after you’ve tightened it, you will relax the muscles in the shoulder and allow the blood to flow into the arms.
 
2) Wrist Stretch – Slowly stretch the wrist backwards, hold for 2/3 seconds, then
Slowly stretch is forwards and hold for 2/3 seconds. Repeat 3 times. This exercise
prevents tightening of the wrists.
 
3) Make a Fist – Hold the arm at right angles from the elbow. Make a fist and tense
it, and the whole of your arm. Hold for 2/3 seconds, then relax and let the arm flop to
your side. Repeat 3 times. This will help the blood flow and tone the muscles.
 
4) Neck muscle stretch – Try to make a double chin, to stretch the muscles at the
base of the neck. Hold this position for 2 to 3 seconds and repeat 3 times. Always
stretch very slowly.
 
5) Lower back loosen – Stand with your feet shoulder width apart  slowly circle your
hips 5 revolutions to the right and then 5 revolutions to your left. 
 
But remember warm up exercises shouldn’t be painful, so if they are stop!
Hutchful concludes “It is also important to take frequent breaks during a Wii session, at least once every 45 minutes for most people, but 20 – 30 minutes for those who do not exercise regularly.  Don’t forget - pain is a warning sign, so if you feel any pain no matter how slight – stop!”
 
For more information, or to find details of your nearest BCA chiropractor, visit
www.chiropractic-uk.co.uk or call the BCA on 0118 950 5950