Without insulation, your house's hot-water pipes act as a gigantic radiator, transferring heat to the air so efficiently that any water in the pipes—even if it left the boiler at a toasty 105 degrees—is barely lukewarm 15 minutes later. So if you wash your hands to prep dinner, then need to rinse a pot, you'll have to wait for hot water all over again.
The solution: Insulate pipes wherever you can reach them by encasing them in rubber or polyethylene foam tubes. The tubes come with an adhesive-coated slit down the middle, so you just ease them over the pipe and press the ends closed. Seal the seams with duct tape. Tests show that insulation can double the cool-down time in 1/2-inch pipe and triple it in 3/4-inch pipe.
Tubes to cover 45 feet of 3/4-inch pipe: $55
Roll of duct tape: $5-10