I own a lot of oxford shirts and some of them get a ring around the collar. What can I do to remove the ring and prevent it from happening again?
The grime you see on your collar is not dirt, it's actually a build-up of sweat along with the oils from your skin. That's why normal washing rarely removes it.
Removing the Ring
Run the collar under warm water and add a dime-sized amount of shampoo or dish detergent (like Palmolive) to the center of the collar.
Take the fabric and rub it against itself, working from the center out to the ends of the collar.
Lather, rinse and repeat if necessary.
Toss the shirt in the regular laundry and it should come out good as new.
Prevent future rings by washing your light colored shirts after every wearing. Run a gel stain remover, like Shout Ultra Concentrated Gel ($3.50 at Drugstore.com) before washing.