After getting married, my husband introduced me to ring-around-the collar:) I learned quickly that simply laundering his shirts did not remove it. I know from other projects that Dawn dishwashing soap is great at getting rid of grease and such, so I decided to give it a try on the dirty collar.
I rubbed in the soap into the dirty collar before throwing the shirt into the washing machine and washing it as normal.
On voila! The ring-around-the-collar was pretty much gone. Another victory against stains and dirt!