Can you please tell me how to prevent hair loss?

1. Qin shampoo, shampoo is the best interval of 2-5 days.
2. NO Smoking
3. Restraint alcohol, liquor
4. Suppress the spirit of the elimination of flu
5. Eat more vegetables to prevent constipation
6. To be suitable for air-conditioning

In addition to chengfengs post, stress can cause hair loss!

It depends on the cause of the hair loss. If it is genetic I really don't know it there is anything that can stop it.
Chinese herbs can treat it if it is not genetic. The usual cause of it in Chinese Medicine is what is called "blood deficiency" which may not or may not show up as anemia but in Chinese Medicine the signs and symptoms will show up before anemia is diagnosed with blood tests.

Stress which is called "Liver Qi Stagnation" is another cause of hair loss.
There are good herbal formulas for both these conditions but a practitioner would have to diagnose you.
It might also be caused from some sort of sink condition. There are Chinese herbs that treat this too.

I've heard that not smoking, eating less meat and wearing a hat while in the sun can prevent hair loss even if you are genetically prone.