Shirt collars are, by far, the most difficult part of the shirt to get clean. While it's not much of a problem with T-shirts, dirty collars on turtlenecks and dress shirts can hound a person for a long time if not properly treated. Thankfully, there is more than one way to get those pesky stains out of shirt collars.
If you have the time and want to make certain that your shirt collars are as clean as possible, nothing beats scrubbing your collars with your bare hands. Depending upon the material of your shirt, you can just use a damp cloth and soap. However, for materials sturdier than simple cotton, you will need a scrub brush and true washing powder.
If you don't have the time for scrubbing your shirt collars but need to get them clean, try direct application of your washing powder to the dirty area of your shirt before tossing it into the laundry. While this may not sound like much, having your cleaning solutions in direct contact with the dirty parts of the shirt give the soap time to soak into the fabric before being washed out. This loosens the dirt embedded in the fabric. Soap interacting with the water lifts much of the dirt from the clothing.
If all else fails, try some home remedies. A popular home remedy involves coating the stained area with a thin layer of baking soda, dampened with some vinegar. Allow this paste to sit for about five or six minutes on the collar before gently scrubbing the area with a small scrub brush or, even better, a toothbrush used for such purposes.