I've also had some success with soaking fabrics in hot water and then washing them.
Sometimes the 'shrunken' fabric is really quite satisfactory. And sometimes it's not.
In the long run, it's probably better to get what you want to begin with.
'Musin' can be purchased in all sorts of weights and qualities.
If you buy the RocLon tea-dyed muslin - it says 'preshrunk' on the end of the bolt. Don't believe it. It shrank a LOT for me~