After seeing Ben Mathis'/Poopinmymouth's tutorial
on how to do flow maps in PaintShop Pro, I realized that there's a very simple way to create flow maps in Photoshop using a simplified version of his method.
First, either render out or copy the cone normal from Ben's tutorial:
It helps if this cone image is the same size as the flow map you'd like to create, or even larger. The larger the better, in fact.
Once you have the cone normal, open it and your flow map as two separate documents in Photoshop and mask out the center of the cone normal:
It's important that you do this as a mask so you can be sure you're able to sample the very center pixel of the cone by turning the mask on and off. Masking out the center ensures that you don't sample directions from the wrong side of the cone at the start of your stroke.
Once you have the cone normal masked, turn off the mask by Shift+Clicking on it, sample the center pixel with the Clone Stamp tool, and switch to your flow map document.
By sampling the cone normal document, you can now stamp normal vectors onto your flow/comb map quickly and easily:
This technique can be used to create flow/comb maps for anything that actually uses them, be it hair, flowing water, or animated particles like you might see in Uncharted 3.