I make sure I wash the shirt only in cold water. Heat of any kind will shrink the shirt. Never put it in the drier.
I hand wash or take the t-shirt out of the washer, while wet, hang it up and gently stretch it by hand. Both hands parallel to each other, going all the way down the sides of the shirt, from the underarms down. Sometimes, I'll raise my arms & place my forearms inside the sides of the shirt and gently spread them apart to stretch that way too.
To stretch the upper chest, place your hands so that they are at the inside edges of the chest area, where the material is still one piece and meets the sleeves seams. You want to stretch one whole piece of fabric. Never pull from the sleeve or stretch across any seams as that will weaken the seams and possibly cause the stitching to rip apart. Do the front chest, then flip and do the upper back.
Stretching the material has to be done while wet. That is KEY. Then a few hours later, while still damp, gently stretch it again, in case it has shrunken a little while drying. Then one last time before it's fully dried.
If that is still not enough, or it shrinks too much as it dries, I take a thick piece of cardboard that will fit inside the stretched, wet shirt, the size I want. I place a plastic bag over the cardboard, then slip it inside the wet shirt while it dries. That will help the shirt retain the stretched shape as it dries.