For nylon halters
Windex All Purpose cleaner (the citrus) or similar, works great as a pre-wash stain remover. I even use it on my turnout sheets for those pesky manure and urine stains. It's cheap and effective.
Spray it on, let it sit a couple minutes. Then wash and scrub stubborn spots. Respray the stubborn ones if needed. I've tossed them in the washing machine this way as well as just hosed them out. Worked either way.
I like the Dawn and fabric softener tips. I'll be adding those to my wash routine.
It's a lot like nuts and bolts - if the rider's nuts, the horse bolts! ~Nicholas Evans