There are a lot of great ideas here. I always check my tension on a sample sandwich every time I change my bobbin. That way I don't start on my quilt and have to go back and pick out nasty stitches. I was having a lot of trouble with stitches being loose on the backside and was also very frustrated. One day I saw a sewing show and the hostess said "Remember to back off your tension (top) one number and sew fast while moving the quilt slow." That was the best advice I have ever got for free motion quilting. You have to concentrate on what you are doing and modify the speed of your machine and the speed that you move the material but it's worth the effort.
Hope this helps some.
Ruthie