Edit: For some reason Rapidshare killed my link, so, i cleanded up the file (now have just the custom shape i'm talking on this post) and upload directly here.
strongly inspired from this video of mr.m@ (check all the thread 'cause this guy rocks) I decide to build this little tutorial that let you easily construct a kind of grid can help on some perspective construction.
First of all I make the grid on Illustrator, pasted in Photoshop and converted on a Custom Shape. You can download the file here.
Well, first of all, you need to import my Custom Shape on your Photoshop's Presets.
Select Custom Shape Tool (U) from the toolbar, than select the Path button on the upper bar (red circled on my picture) to let the shape act as a Path, and finally load the CA's vanishing point.csh in the preset manager.
You'll see appear on the bottom of your Custom Shape Picker (don't worry if the thumbnail seems blank, it's regular).
Trace your Horizon Line whit a guide, put the cursor on that line everywere you want and than trace you Custom Shape holding ALT (to create the shape by his center) and SHIFT (facoltative, if you want to keep the proportions).
Now you have a grid of path lines that you can easily hide deselecting the WorkPath from the Path palette, or just pressing CTRL+H that let you show/hide all extras selected on the view>show menu (also the guides).
If you want to draw another Vanishing Point you just have to use the Path Selection Tool (A) (Black Arrow) and drag everywhere holding ALT key (to duplicate the path instead just moving).
Obviously you can draw a single vanishing point inside your cavans.
You can see that the paths are ever visible because Photoshop invert the color (wow, what unexpected cool effect on that pic! ;-])
If you have to adjust a single line of the grid for your purposes, you can easily doing whit the Direct Selection Tool (A) (White Arrow)...
...dragging a single path's node.
If you like more working whit layers, you can create some adjustment layers using our path like a mask (just stroke the path whit an appropriate small brush in the mask).
To have the same visual effect we can use the invert adjustment layer, but if you want to isolate single vanishing point you can use simple fill layers whit the color you want, or in many other way.
The only nitpick is that you can't edit the path after converting in layer.
I hope that's useful, many thank to m@ for inspiration.
If someone want correct my english, I'll be happy ;-]