Use a piece of plain cardboard ( like the side of a computer box, not a cereal box) on top of your ironing board and under an appliqué piece when you are turning the edges under with an iron. The result will be a firm resistant surface for a smoother edge on your appliqué piece.
Always lay pattern pieces for appliqué work on the bias before cutting out.
Always use pinking shears to cut out your appliqué pieces. Pinking shears will clip "in-y" curves for you and take fullness from "out-y" curves all with one swipe. Only very tight "in-y" curves will need to have additional clipping. Be sure to use a new pair of aluminum pinkers like the Fiskars brand. Do not use the old heavy pair your grandmother left you. Your hand will become too tired from the weight and stiffness of an old fashioned pair.
When your pinking shears become tight or stiff, use some machine oil at the joint and along the zig-zag blade edge. Wipe away excess oil with a paper towel or tissue and then cut through a scrap of 100% cotton or wool to insure no oil residue will appear on your good fabric. Microscopic fibers seem to stick to all the metal surfaces of pinking shears and the buildup of fibers prevents fluid movement of the blades.
When preparing very tiny appliqué pieces such as small circles, dots or berries, use a washable glue stick to hold turned edges securely in place.
When an appliqué piece has an "in-y" point like the cleavage on a heart, use Fray Check to seal the cotton fibers before you clip into the point to turn under the edge.
Use short pieces of 1/4" quilter's masking tape instead of pins to hold an appliqué piece in place while sewing. This way there are no pins sticking up for your thread to get tangled around while hand appliquéing and there are no speed bumps as you machine appliqué over the pins. Use a pair of serger tweezers to lift away the pieces of tape as you come to them to avoid a gummed-up needle.