Birch bark is a lovely material that can be woven, folded, and lashed into a great variety of projects. If done mindfully, birch bark can be sustainably harvested without permanently damaging trees.
The images in this Instructable were taken by Alex Kamerling at Week in the Woods: http://weekinthewoods.org/
Step 1: Tools
Bark may be harvested without any tools, but it is nice to have a short sharp knife, ladder, string, ruler, straight edge, soft wax, and scissors.
Step 2: Time of Year
Birch bark should be harvested in the spring when the sap is running in the trees. In Alaska and Minnesota (and presumably many other places) the best time to harvest birch bark is when the wild roses bloom. When done at the right time of year the bark will literally pop off the tree. However, if done too late in the season the cambium (inner bark) will come off with the outer bark. If the cambium is damaged the tree may die. If done right, a tree will re-grow its bark over the course of 10-20 years. I have heard that the best bark comes from trees that have already been previously harvested, and traditionally people would return to harvest in the same grove over the course of several generations.