Make sure the t-shirts are 100% cotton or don't use the fusible stabilizers.
If you are using fusible, use a pressing cloth so as not to overheat fusible & be sure to carefully follow instructions about iron temp.
Sulky makes nice cut-away stabilizers (both fusible & sew-in). Whatever brand you do buy, it needs to be cut-away stabilizer so it will stay adhered to fabric when used or washed.