Slacklines are popping up all over the place.  The National Park Service worked with local "slackers" to create a slackline policy:

The Rules:

• All lines must be constructed so as to protect trees from any damage
• Lines may remain in place within the Camp 4 boundary for the length of the owner's stay
• All lines constructed outside Camp 4 must be removed when not in use
• Lines must not be constructed on oak trees in Yosemite Valley
  

The Reasons:
Anyone who has spent any time learning the art of slacking knows how damaging these lines can be to trees.  There are many ways to pad and protect the anchor trees (carpet, sticks, sleeping pads, haul bags, clothing, etc.), but make sure whatever you do is working.