I'm currently also working on my squat form, and Figure 2-8 on PDF page 18 of Rippetoe's book is instructive.

Basically, I found that to really work the glutes, I needed the hip angle (as labeled in diagram A) to be as small as possible at the bottom of the squat. This requires leaning forward at almost a 45 degree back angle at the bottom of the squat -- otherwise I can't achieve as tight of a hip angle (as shown in diagram B, if I were to remain mostly upright). As you rise, the hip angle straightens up, as does the back angle, until you're all the way upright. The key is that the back angle is only needed to not fall forward; the large change in hip angle is what you're trying to achieve.