Be careful about the type of iron-on interfacing you buy. You want the thinnest and softest you can find. Buy small amounts and experiment on scrap t-shirts to make sure you like the result. You want the drape to remain as soft as possible. Some interfacings end up making the fabric quite stiff.
The iron-on interfacing I like to use is a tricot knit interfacing because it is soft, has some stretch, and leaves the fabric with a decent drape. I think places like JoAnn's carry it, but it is much less expensive purchased by the yard online from a drapery place. (I Googled to find it.)
There are books on making t-shirt quilts that contain helpful hints about interfacing, sashing, etc. Check Amazon for titles.