Actually, B36, I just re-read your message and have a bit more advice for you.
First off, 'hair thickening' shampoos are sold over the counter and make snake oil-like claims. Some of them seriously have a pine tar-like substance in them which supposedly delays telogen hairs from migrating out of their follicles and makes the hair look thicker. I would be wary of these, as I would think that keeping hairs IN the shaft would probably also keep minoxidil from getting IN.
If you are looking for a true, long-term thickening shampoo, then some studies show that the ingredient in Nizoral helps thicken shafts as much as an over the counter, non-pescription shampoo can. Second, although I have yet to try it, many here swear by Folligen products that supposedly do the same thing. I would recommend heading to the "Products" forum of this website and looking at what people have to say about these two products.
The product I recommended, Redken Extreme Anti-Snap I would not necessarily recommend as a thickener per se. For me, and the hair condition I had in my thinning areas, it makes my hair feel and look ten times more healthy. It didn't literally add body to my hair, but by making the stunted thinning hairs less brittle and more healthy, I have found that it makes these affected hairs look more like the other, non-thinning hairs on my head and gives my hair more healthy uniformity of texture. Before, the hairs on the thinning area would curl up and have an unhealthy texture. Now, this problem is gone.
Hope this helps.