Tutorial: Change rest pose to T-pose in Blender (manual posing)

(Author: Jonas Hauquier) This is a very quick solution to setting your mesh to T pose in Blender (This is the approach with manual posing). I assume you have imported a rigged human model into Blender (for example using the MHX format). 1) In pose mode, pose the armature so that the human is in T stance 2) Exit pose mode, and go to Object mode. 3) Select the human MESH (not the armature) 4) Remove any shapekeys. Blender does not allow modifying your mesh with them, so they will have to go. In the shapekeys panel press the "-" icon until the list is empty. 5) in the Modifiers panel (which is usually to the right of your screen), click Apply on the ARMATURE modifier. 6) Add a new Armature modifier to the stack (click Add modifier) and attach it to the correct armature again (select your armature from the Object dropdown) 7) Select the armature again, go to Pose mode again. 8) Press SPACE and search for "apply pose as rest pose", hit enter Now your armature should be in T pose, and your basemesh should fit the T stance. We have not applied any special geometry deformations, so posing the armature with arms down should give realistic joint geometry around the shoulders.
via YouTube http://youtu.be/r1AkueG7zpY