Check out a book called 'The Baldness Cure.' It gives a ton of scientific and nutritional information about hair.
It tells you the KIND of saw palmetto to use...(I think it has to be at least 13% fatty sterols to work)
I would also recommend zinc (for a year or so), omega 3-6-9, and multivitamins.
Some genes are stronger than others and you really have to pull out all the weapons to fight hair loss if you really want to keep it, but I think its possible for almost everyone to keep their hair if they start early enough and are dedicated enough.
Personally I am still losing hair after 2 years on propecia but its slowed down SIGNIFICANTLY and regrown in a lot of areas (except the receded front, which I have lost all hope for

)
GOOD LUCK!!