For your stitching to disappear you want a thin thread, which is why silk is often recommended, but a thin cotton is wonderful too. One of the best "inventions" is the Frosted Donut by Superior Threads. It is a bobbin holder filled with 35 or 36 different colors of Masterpiece thread (Alex Andersons favorite). Since you don't use large amounts of thread with hand applique but often need lots of different colors, this is perfect. I also use Mettler 60/2 cotton thread, and DMC.