I'm pretty new to manga and I am just kind of working through various tutorials. One thing I did find rather helpful is the ability to use Sketchup to create background scenes for images. It seems much easier than working with perspective rulers imho.

Here is an example where I created a quick background dojo (texturing it with Gimp) into a simple scene.

Anyway, I just thought I 'd mention it as a tool which people may want to explore on their own.