Keep a supply of testing fabrics ready for use before each project. If you do a lot of quilting, keep some "sandwiches" ready as well. Whenever I start a new project, I always like to run a couple lines of stitching on the fabric, or a similar fabric, just to make sure that my tension is right, as well as my stitch length. I have found that different fabrics usually require a little tension adjusting. This is also a good way to try out new stitches for sewing, or for practicing your free motion quilting. If you have them ready to go, you will be more apt to test first.
There are also fabrics that I don't use very often, such as organza, satins, and linen. This is when it is most important to have some testing fabrics ready. If the pattern doesn't leave any extra for testing, it's well worth it to buy an extra eight of a yard for practicing or testing.