If you are at work and just now noticing in horror the yellow stains in the armpit areas of your dress shirt or the dingy collar, keep your suit jacket on and pick up a bottle of bleach on your way home. When it comes to white dress shirts, simply throwing it in the wash with the other clothes isn't going to cut it. You need to add bleach when washing whites to get rid of inevitable stains. Bleach will also get your dress shirt even whiter than before.
Start your washing machine for a small load and set to warm water.
Add a 1/2 capful of detergent to the water as it is filling up the washer.
Add 3/4 cup bleach to the water.
Wait for the wash cycle to begin, then add your white dress shirt. This ensures that the bleach is thoroughly diluted so it won't put holes in your shirt.
Remove the shirt from the washer when the cycle is finished, then place on a cushioned hanger to air-dry.
Iron the shirt with a steam iron before wearing.
If you have a front-loading washer, you will typically need less bleach. Consult with the manufacturer manual to determine how much bleach is recommended for a very small load of laundry.
Read the label of your dress shirt carefully before using bleach, as some might instruct you specifically not to use chlorine bleach.
Be very careful when handling bleach as it can burn your skin. Wear rubber gloves if possible.