There’s nothing worse than starting a sink or toilet repair only to find that the shutoff valve won’t shut off. Some shutoff valves are easy to replace. For those that aren’t, turn off the main water valve, remove the packing nut, and then unscrew the stem and take it to the hardware store to find a replacement washer. Clean any grit out of the valve body and pop on the new washer. The valve will work like new.
How to DIY it: This job can be messy, so protect nearby surfaces by covering them with plastic or cardboard. Spray the springs with garage door lubricant (about $7 at home centers). Don’t use oil, grease, or other lubricants. They may be cheaper, or you may have them on hand already, but they won’t work as well and tend to pick up dust and grit—just what you don’t want on moving parts.