Mark an inch on your finger. You'd be surprised just how useful this can be for certain tasks around the house. Sewing, for instance, is made that much easier. Measuring things (obviously) becomes a piece of cake when you have the actual measurements on your hand; there is no need to scramble around for a ruler. This is just one of those straightforward and practical life hacks that'll actually help you out.
Just be careful not to get the measurements wrong, or else you could be looking at a whole lot of trouble. It's important to be as precise as possible when it comes to these things.