I am sure you will get some good, and some less acceptable, suggestions for preventing hair loss - but let me suggest at the very beginning that the very best way to slow (prevention is much more difficult) is very good overall nutrition and health.  This means getting a wide variety of plant based foods, with minerals and vitamins of which most people have some deficiencies.  The other is having great sleep habits and exercise, and proper water for hydration (not sports drinks as the sugars tend to lead to diabetic type damage even if the person does not get full blown diabetes).  When the scalp is receiving a good blood supply, with good nutrition (not just sugars and oils in abundance), the follicles will be healthier and will make better quality hair.  Without the stress, the hormones will also be in better balance.

^ Absolutely agreed.