Question
Is there a way to stake a 15 foot tall aspen tree with a 6 inch trunk - that is starting to lean as a result of wind?

Answer
Sometimes tree just grow in a lean. Check the ground on the side away from the lean--if the ground seems to be pulled up by the tree then the roots of the tree are damaged and you may as well take it down now. BUT if the ground seems level and not up heaved then the tree is just growing as leaning tree and it is not a health problem. The tree over time will grow more straighter and more then likely have a bend in the tree as it tries to grow up right.

IF you must you can stake the tree. Once you straighten a tree, it's a good idea to stake it. Drive a couple of wood stakes on the side of each tree. Then run wire or rope from the stakes and wrap it loosely around the trunk about two-thirds of the way up. To protect the bark against chafe, cover the wire or rope with scrap pieces of garden hose. You shouldn't let your trees become too attached to their stakes. Unstaked trees develop stronger root systems than those with artificial supports. I would leave the stake fro a year to give the roots time to grow back.