Although this is not necessarily a sewing tip, it sort of still is as it helps you to prep for your sewing in the best way possible and ensure that the project stays the way you designed it even after the first wash. A lot of people make the mistake of only washing their fabric for the first time after they finish sewing a garment and they don't know how catastrophic that can be for the end result.
Some fabrics are very prone to shrinkage, especially cotton. And one of the worst things you can do is finish a sewing project only to find that it is a few sizes too small after you wash it once.