By decal, you mean a patch or logo on the article? If so, you cannot really remove it without it showing. The best thing to do is to buy another one to go over it or make your own. You can buy fabric that meshes to fabrics together with an iron so that would be how you make your own.
Wouldn't you be left with a gummy unattractive look to your shirt even if you were able to remove the decal? You could sew on a fabric applique over the decal if it's the decal itself you do not like. (10/05/2008)
Keep in mind that if the fabric has faded at all, it will not have faded underneath the decal and the outline will remain. If you still want to remove them, put an old white cloth over the decal and iron the heck out of it. The decal should stick to the cloth. (01/23/2010)
Apply fabric softener directly to the "iron- on". Allow it to soften the adhesive, then cover with a towel and iron on "hot" till the iron-ons are loosened. However, the "shadows" will not go away. (09/07/2010)