First off, sorry for your problem. I know it MUST suck.
I also suffer with THIN hair at the base of my head because I always put my hair in a ponytail. The result? My hair looks like shlt whenever I try to wear it down. So I'm correcting that problem now.
We both have thinning hair, so THIS is what has worked for me:
1. Shea butter wash (1x a week)
2. Castor oil "rinse" (1x a week)
3. Henna (2x a month butas a guy, I don't know your thoughts on henna)
In depth, here is how you do the shea butter wash: riarankine.hubpages.com/hub/Thicker-Hair-with-Organic-Shea-Butter-Wash
And if you're consistent with all of these hair thickening treatments, you WILL see results.
3 Natural Ways to Thicken Your Hair: ezinearticles.com/?3-Natural-Methods-to-Thicken-Fine-Hair&id=7956667
Be cautious of the hair growth vitamins you take. I haven't heard about minoxidil, but I have heard that most stop working when you discontinue use, AND you lose the growth you gained with it...I don't know WHY.