The key to making an autograph on a shirt or hat last as long as possible is to use a permanent marker like a Sharpie. A marker used to label laundry is even better. It's made to be permanent ink on fabric.
Whatever pen is used, you can set the ink by using heat. Place a clean white cloth over the signature and iron at the highest temperature acceptable for the fabric type. Use a dry iron - no steam - and press for at least one minute.
When you are ready to wash the garment, turn it inside out. Use only warm or cold water and a gentle detergent. It can be dried in the dryer or hung to dry.
With a bit of care, you'll relive the memories for years to come.