B Vitamins are supposed to promote healthy hair. Also drink plenty of water and avoid using heat styling tools on your hair without a protectant. Don't wear your hair up, a ponytail holder can break your hair. Wash your hair every other day to leave the good proteins and oils intact. Use a comb and not a brush when your hair is wet to prevent breakage.
I love Chi Silk Infusion and Redken AntiSnap. It makes my hair shiny and manageable.
Also, these products are top selling for haircare and treatment- link
this is what I do... my hair doesn't usually not very easily. so 1) I don't brush it every day. messy hair gives your hair a thicker look.. like ke$ha! 2) if you spray a little hair spray on the bottom part of your top layer, you can scrub it in (technique-like scrub, like scrunching it in) to your scalp and it with give your hair some volume. also, flip your hair over your head, give it a few puffs of hairs spray.. scrunch your hair and mess with it and it will add volume forsure :)
make sure you don't wash your hair everyday with shampoo because its actually really bad for the hair like you can just wet it , so like everyother day , and try curling it it looks way thicker when hair is curly or wavy , you can also try some products from any store it says on the bottle that's its for thicker hair , I heard it actually works most of them :)
My dermatologist suggested that I use this product when I had problem with hair loss from stress. I in turn suggested it to other people and so far everyone was happy with the results. The name is ALPECIN with caffeine. It's supposed to help the roots of the hair be stronger, less hair loss and better looking hair. I don't know if it can be found everywhere, but look for it in pharmacies. If you can't find it something with caffeine is good anyhow.
Try hair and mane shampoo. It really works. You can do a google search on it and read all the reviews. I've never tried it personally but people say it helps to make your hair grow longer and thicker and healthier.