100% Cotton t-shirts, though usually pre-shrunk, will still
shrink a small amount, especially if washed and dried at hot
temperatures. Suggested care to prevent shrinkage is to avoid
washing or drying at the hottest temperatures.
If decorated, bleach is not recommended for t-shirts. Likewise,
washing inside out is a good idea to protect designs. Ironing
directly on designs is not recommended for most decoration
methods. For designs put on by some decoration methods such
as silkscreen or flock you can iron the opposite side of the
shirt at a low setting.
Polyester t-shirts should not be bleached, decorated or not.
High heat can also be damaging.
Silk t-shirts often have to be dry cleaned though some may
be hand washable. Check the label for care.