I’ve got a soccer jersey (polyester) that’s just a little on the tight side. Are there any good tricks for stretching out a shirt?
wear it
I was thinking of the same problem today, when a shirt I ordered online turned out to be a hair too small…the best plan of attack I came up with is to keep wearing it and never put it in the dryer. Unless you’ve got a fat friend you can blackmail into wearing it for extended periods of time.
Fill it with books (or nearest large heavy items) and leave it?
Third vote for wearing it (particularly while sleeping) and then washing it and letting it air dry. Certainly works on my bras.
Cite? 
Wet it and stretch it over something, like a small overturned garbage can or pail, and let it air dry around that that.
You need two dogs, one small one with issues and a large breed of your choice.
Pay a canine behaviorlist $100 to visit your home and analyze you and your dogs.
Put the t-shirt on the smaller dog to help it with its issues (http://www.anxietywrap.com/) and voila! The larger dog will drag the smaller dog around by the t-shirt and stretch the hell out of it. 
For a serious answer, I have read that you could try putting a bit of hair conditioner in the rinse cycle and then stretching the shirt out into the desired shape, and leaving it to air dry.
Something about the conditioner allowing the fibres to be more slippery & therefore easier to stretch.
I’ve never tried it. YMMV, etc.
I used to have a great t-shirt collection. Problem is I stated going to the gym and went from a medium to a large or xl. So I’ve tried everything:
Stretching by hand
Running over it with my car
Vinegar
Fabric softener
Air drying
The bottom line is some shirts will stretch out with time and wear and some (most) never will.