It depends on how much you want it to shrink...but it it's just a little loose, you could steam it, that generally causes most hats to shrink unless you have them on some sort of form to hold their shape. If it's too large for that to work, you can take double-sided foam tape and stick it on the inside of the hat band (between the band and the hat itself). You can also use this to pad the hat band if the shape of your head doesn't conveniently fit the shape of the hat (some hats are loose on the sides for a lot of people). In a pinch, you can do the same thing with paper towels...the tape just doesn't accidentally fly out at inconvenient moments... If it doesn't have a band on the inside of the hat...well, you probably won't be wearing it long, because the straw will start poking into your scalp and get uncomfortable in a big hurry!