Summarize:

No two dandruff shampoos are exactly alike; similarly, no two dandruff sufferers are exactly alike, either. The best dandruff shampoo for one person may not be the best one for you, so be prepared to try out a few different options before finding one that works.  Zinc pyrithione shampoos tend to have fewer negative side effects and are generally well tolerated, making them a popular first option. If you have darker hair, tar shampoos and selenium sulfide shampoos are equally good options as long as you follow the label instructions closely. Since salicylic acid shampoos can cause additional dryness, they are generally a less popular choice, but are still safe for most people. Ketoconazole shampoos are generally used when other anti-dandruff shampoo types fail to work or stop working. For a more precise recommendation suited to your hair type and scalp needs, consult a dermatologist. You should always follow the label instructions closely for any dandruff shampoo you use, but generally, you'll need to apply the shampoo to wet hair and thoroughly massage it into the scalp using your fingers. In most cases, you'll need to let the shampoo sit on your scalp for about five minutes before rinsing it out. Others, however, should be rinsed out immediately after application (especially those containing selenium sulfide). To bring severe cases of dandruff under control, you'll typically need to use your anti-dandruff shampoo once every one to two days.  The exception is Ketoconazole shampoo, which should be used only twice per week.  As your dandruff subsides, you can cut back the amount used to about twice per week, or once per week if you have coarser hair. One shampoo may work for a period of time before suddenly becoming less effective. If this happens, consider switching back and forth between two different ingredient types, using each one at least once per week. You may also consider switching entirely to another type of dandruff shampoo instead of alternating between the old and new ingredients, but doing so may prove less effective than alternating between two.
Commit to trial and error. Massage the shampoo into your scalp. Use the shampoo regularly. Swap between two anti-dandruff shampoos, if necessary.