I like thin thread. My favorite is Aurifil 50 weight. I like it because it's thin, strong, and comes in any color I can dream. My only problem with it is that it is in short supply in my small town and I have to acquire it at quilt shows or drive an hour or so to obtain it. And I have done that drive. More than once... but I digress. I do use other manufacturers and weights but I like the thin thread because it's forgiving. If I scribble-sew all over, mistakes aren't obvious. With a thicker thread, it would be. But thick thread has its place. I like to cover things with thread because I think it gives an organic look. My favorite subjects have hair or feathers. But thick thread is really great for trees. And for less organic things.
With my thin thread, I use small needles. I like 75/11 or 80/12 sizes. I usually use Schmetz jeans/denim or Microtex. The shop where I purchased my machine suggested a new brand, Inspira. I have been using their denim and Microtex and sometimes their titanium-coated needles and they seem to work pretty well- they're cheaper too. I change my needles a lot. When they make that popping noise, it's time for a change. Sometimes I will notice nests or loops on the back -- time for a new or different needle. I don't change my machine tension a lot after I make my sample. Just the needle. When using thicker thread, a larger needle is in order -- without a lot of blather about technical stuff, I suggest 90/14 Topstitch needles.
As for fabric, I use anything I can lay my hands on. My favorite stuff to use is hand-dyed cotton, but I'm not proud. I'll use anything that suits the project.
Which brings me to stabilizers. I have been using Decor-Bond for a while now -- it's fairly lightweight and doesn't feel too crinkly inside of a quilted project. A friend recommended Sulky Totally Stable recently, so I'm going to be experimenting with that this month. I have a project that I abandoned because I didn't use any stabilizer before I started covering it with thread and it puckered. I'm going to see if I can reclaim it with stabilizer. Sometimes I use batting and sometimes I don't. I like thin batts when I do use them.
And don't forget feet: I use my freemotion foot with the feed dogs down. Sometimes I forget and leave them up -- I usually don't notice until I remove the piece. So I'm not sure it's all that important.
Next: thread-sketching and the value of making samples