You mentioned that you have been "tested for everything" but what sort of doctor? I'm not offering medical advice, but there are many health care issues which can cause hair loss (for example, thyroid issues, low vitamin levels, etc). Is your diet lacking in any areas? Also, what I think is most important, how much hair are you loosing and do you see any bald spots? I only ask because I am dealign with hair loss (due to a health issue) and when I brought it up to my specialist, he was kind but did point out that I have long dark hair so even a few strands look like much more than short strands. If you want to get really intense about it, a friend of mine actually collected all of the hair she lost each day (or as much as she could) and put them in envelopes. Then, she counted the hairs at the end of one week.
Since my hair loss is due to a health issue, sometimes it's worse than others. When I'm going through a time when it's worse, I do not dye my hair (I rarely highlight anyway, and when I dye I tend to just do a glaze which is less harmful than lightening), minimize heat styling (when my hair is fine, I usually only heat style 1-2 times a week at most), and generally just baby my hair. Another friend of mine was noticing hair loss around her face, and her stylist said it may be due to how she wears her hair - said friend is a fan of tight pony tails, and that the stylist believed was causing the thinning around her face. My friend started putting her ponytail in different places on her head, as well as not going for the tight ponies and trying out other options. Within 6 months, no more issues.
I've tried several products for hair loss, and most I found no noticeable difference or the difference was so small that it wasn't worth the price (very few of the products are cheap!). Aveda's Invati shampoo and conditioner offered little to no difference, plus I just didn't like them (I went through the bottles, re-purchased adn then was done). Plus the shampoo is so thin that I felt like I wasted product, and the conditioner I felt weighed down my hair. I even tried it with the scalp spray, and that offered a bit of a difference. I kept up with the spray after I went back to my old shampoo and conditioner. The difference wasn't worth the price for me, so I tried Alterna's and liked it much better. I did notice a small difference, and while my mom couldn't tell what the difference was, she did think that my hair is looking better. When I was getting low, I tried L'oreal's version and while the price is much more friendly to my wallet, I didn't like it as much. I wanted to like it as much! The scent is a little more masculine, but I found it didn't last all day and it did tingle a little on my scalp (Alterna's doesn't for me) - not bad, but worth pointing out. I did think L'oreal's worked better than Aveda's but I would rather shell out for Alterna's.