When doing heavier quilting, it helps a *lot* to use a finer weight thread.
Batting makes a difference too. For heavier quilting, a traditional batting such as Mountain Mist Blue Ribbon 100% cotton (which requires quilting lines no further than 2" apart) will end up softer than something like Warm and Natural (which can have quilting lines as far apart as 10"). Some of the thin poly batting will stay softer too.
I once did stipple quilting on a doll quilt. That's how I learned that regular 3-ply cotton thread can make a quilt almost stiff as a board (even with MM Blue Ribbon batting). The biggest thread I would use for heavy quilting on a regular size quilt would be Aurifil 50wt 2-ply or a 60-wt polyester thread (such as Superior's Bottom Line or Fil-Tec Glide). If your machine can handle it, you could go all the way up to 100wt silk thread for quilting -- which is very, very fine. Connecting Threads has a 70wt poly (if your machine can handle it).
Last edited by Prism99; 01-15-2013 at 10:21 PM.