Shoelaces stretch out over time and end up being too long to be single knotted, which is an annoyance most of us just put up with. Instructables user 2xlbu4u found a simple way to shorten laces without ruining them.
With a small piece of cheap heat shrink tubing, you make a new plastic tip and then cut down the laces to your desired size. Once the plastic is where you want it, shrink the plastic nub on with the heat shrinker. The end result looks exactly like a new pair of laces would, but with a custom length that will fit your shoes properly. Find the guide over on Instructables with a few added tips to getting it right.