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If you want to make the stain easier to remove in the future, do what you can to make sure it doesn't set. As soon as you notice a stain forming, treat it. Don't put the shirt in a dryer if you notice the stain hasn't been removed as much as possible. In general, do whatever you can to treat the stains before they become too dark. The collar stain is the result of grease and sweat mixing together in a bad way, so one way to prevent the stains from forming is to make some adjustments to your hygiene routine. Shower more often, use roll-on antiperspirant on your neck, or use baby powder on your neck to help absorb the grease and sweat. Some shampoos may mix badly with your unique body chemistry. If nothing you do seems to help prevent the stains, try switching to a different brand and style of shampoo. Lean towards using white shirts, instead of colored shirts. The stains may be more noticeable more quickly, but they're easier to treat. As long as you have a white shirt, all you have to worry about removing is some of the grease. After that, bleach can be used to remove the rest, as well as the stain. It is possible to purchase sweat strips, which are stickers that go on your collar to help prevent stains. You can make your own too, if you're crafty or know someone who is. Add a snap-in, button-in, or Velcro strip to the collars of the worst offenders. These can be removed and washed as needed.
Don't let the stain set. Change your hygiene routine. Switch shampoos. Use white shirts. Make sweat strips.