Save your non-LEGO stickers!

Stickers are a way to add additional details to LEGO (and other brand) bricks and easily bring back the brick to its original state if needed. Some useful stickers that come to mind are the leftovers of postage stamps (those can often have long, thin strips of color), return shipping labels (those have letters and numbers you can use in your MOCs), or white stickers (of which you can use permanent markers to make them every color you want).

One possible use is for putting a thin colored sticker on a LEGO windshield to simulate a car's A/B/C/D pillars. This also allows bringing the windshield back in its original state by simply removing the sticker.

Another use would be for stickers with numbers: you can use the numbers as part of your MOCs. Here is an example, but not my best work: