How can I wash a sweat stained baseball cap?

Hand wash it in the sink with shampoo.

I have seen vinyl coated wire frames to place your baseball cap in, then you may place it in the dishwasher or the clothes washer. I just toss mine in the clothes washer, then after I take them out, using rubber bands, I secure the bill to a one pound coffee can to dry.

I spray mine with Shout and place in the dishwasher. Works great!

There are many retailers of baseball cap forms that are designed to help keep the hat shape and then you just wash in the dishwasher. Sometimes, you will need a stiffer form to attach the hat to while the bill dries, in order to keep that shape.

I just throw it in the washing machine it usually comes out fine but I use a HE machine

My husband just had a badly stained cream colored baseball type cap and I sprayed it good with a pretreat for laundry, something like Shout, and let it set for a while then I put it in the washer with a couple of loads back to back, once in the white load and again in with the light colored clothes, and it came out pretty good.