Toss it in the washing machine. Took a good 6" or so of girth off my last one.
I've washed mine several times since getting it, but always in cold water and hung up to dry with no adverse shrinkage for that very reason.
Did you wash yours it in hot water and machine dry it? 
If so that might be sufficient in my case, although if it shrank it would be all around in length as well as girth so it may not be the best option. 
I think cold/cold then machine dry. I wash everything cold/cold when I pay attention to what I'm doing. After getting it out of the drier and realizing it didn't fit at all, I read the tag where it said "Dry clean only" .. too little, too late. I mention it in jest.
If you have sufficient space in the arm holes, use some safety pins to pin off an inch from each side evenly on the seam under the armpits. If that separates it enough in front to be usable again, sew that 1" down the sides (inside out). If you don't cut and re-sew, just sew it on the inside - and it's not too uncomfortable that way, and you happen to "expand" again, you can just grab a seam ripper and expand it back out. Or if you have a seamstress who can take out the existing seams and do all the seamstress magic to resize it down horizontally, that's a better option than my hackish advice.
