Here's that perspective demo I've been promising for far too long. I feel a little odd presenting such a thing, as there are people who are better at it and have better presented their techniques.
I just want everyone reading this to know that perspective is a craft. It's learned both by the mind and by reflex. It's not magic, you can do this. This only real trick is that if you practice enough that you can eyeball minor stuff and things really speed up. Just keep practicing.
If you can spare $40, buy this DVD it's great: [link]