Press your seams. Don’t just press open, but press all machine-sewn seams flat (each side) before pressing open. I didn't know that for years, and neither do many sewers I know. It smoothes out the stitching.
Kendra Van Cleave
Press, press, press.
Pin and press at every step.
By far and away, the thing that would help most costumers move to the next level would be to learn how to make the most of their irons. If they learned how to use things like pressing (tailor's) hams, needle boards or pressing cloths they would see a world of difference in their final products. I have seen classes offered at costume conventions and the like, and these would be well worth the time for most anyone, even intermediate or advanced sewers, because you would be likely to pick up many tips.
A simple and very useful tool most anyone can make for themselves but you don't see in fabric stores is a wooden pressing strip. It is a two to three inch wide strip of hardwood that is between 3/8 to 1/2"(~1cm) thick and about a foot (30cm) or two (60cm) long. The edges on one side of the board can be sanded down giving a smoothly rounded surface. When you iron the seams open on some fabrics like satin and wool the edge of the seam allowance can make an impression on the right side of the fabric. When you press over this piece of wood the iron can only put pressure on the very center of the seam and not the outside edge of the seam allowance and you can get a lot of pressure on the seam (see below). If you need to use steam while you press, cover the wood with aluminium foil.
All you really need is a sander. Most DIY stores sell pieces of 24" (60cm) long oak or maple hardwood of the correct thickness and width. They may even cut it to length for you; I recommend 18" (45cm).Lynn McMasters
Pressing is as important a part of sewing as the stitching is; you really can’t press too much. My twopenceworth to add to the tips above is this: consider investing in a steam iron that has a special steam button that you can press exactly when you want steam. Mine is a Godsend!