One of the best things about the modern embroidered patch is the fact that most of the time it comes from the manufacturer with an easy to use iron-on the backing. This backing allows the patch to be mounted on just about any fabric with just a hot iron. A couple of exceptions to that rule include leather and vinyl; the glue does not adhere well to these surfaces but does a very nice job of leaving stains and burning the surface.
When you do mount iron-on stock patches to cotton, denim, and other fabrics, most of the time they will remain very secure for years. But if you have an exceptionally large patch -- a 12 inch back patch would be one example -- it's a good idea to reinforce the iron-on backing using a needle and thread. But before you panic, it doesn't require you to stitch all the way around the patch until your fingers fall off. You can apply just a stitch or two in strategic locations and you'll be set to go.
If the patch is circular put just a single stitch or two at the twelve, three, six, and nine o'clock positions. If it's a square, rectangle, or triangle, you can put your stitches in the corners. If it's a die-cut patch of no particular shape just pick four or five strategic spots around the perimeter and apply your stitches.
By using the needle and thread you'll reduce the amount of movement the patch makes when the fabric moves and thus, reduce the likelihood that it will fall off. If the patch ever does come loose from the glue the thread will at least keep it in place until you can apply a more permanent solution.
