Don't brush when dry, brush when wet.
Correct, balanced diet (the goodness comes from the inside and will show on the outside).
I am a strong believer in plaiting, loose plaits and this WILL create a thicker, stronger mane, I do this with my colts tail and have done for many many months and the difference in his tail is beautiful.
I'd try not to wash it too much, Natural oils are far better then products.
This works wonders...
Wash the mane with a good shampoo and conditioner. Put in detangler/brush it after the mane is dry and braid it into small sections. You can leave the braids in for about 2 or 3 weeks, depending on how clean they stay. Then repeat.
My friends horses mane is down to his shoulder now. And VERY thick.