These kind of stainings are very difficult because to recieve a good result it depends on factors like quality of fixation and chemicals you use.
In J.A. Kiernan's "Histological & Histochemical Methods: Theory & Practice", 2nd Edition, Pergamon Press,pp 323-324, you will find a recept how to do this staining. I would recomman to do a Immunohistochmical reaction with Anti- GFAP. The Glia-Fiber-Acid-Protein is a structure protein which is highly specific for Astroglia. In the attachment you will find an imge of a stained rat cortex. Immunohistochemistry is easy to handel and garantees you a better reproducebility.