based upon what I read here: http://www.sewgirls.com/sewing/Chiffon-Fabric-5714-.htm
in order of safety try the following:
1] wet and hang dry
2] cold to warm iron (barely warm)
3] warm iron and a pressing cloth
Be careful not to shin the fabric, that means you are staying in one spot to long, or have the iron to hot. It also means you likely have to buy a new garment.