I use starch to stabilize fabric for certain applications -- especially machine applique and when piecing a quilt out of flannel. Heavy starch (1:1 solution of Sta-Flo and water) stabilizes the background fabric sufficiently for machine applique so that I don't have to use anything else, and the starch washes out later. Flannel has a tendency to stretch and distort when piecing, so again heavy starch comes to the rescue.
In the past I have stabilized knits (such as t-shirt fabrics) with fusible nylon tricot, which worked well. You need to pre-shrink the tricot by holding a steam iron above it for a few seconds; you can see it shrink before your eyes! An advantage of fusible nylon tricot is that it has some "give" and does not make fabrics as stiff as many other fusibles. It does, however, add quite a bit of thickness and weight.