I feel pretty comfortable with my lighting, but sometimes I'll run into a situation that just doesn't work as planned. Just today I was trying to set up for a fairly complicated shoot that didn't work out so I'm postponing it.
As I said, your flickr is what inspired this thread. You are very proficient at lighting, and your results seem so natural to you - it is very evident that you have a remarkable understanding of light.
How do you think of light? I mean, do you think of it like a fluid, or like "paint"? Do you consider the give/take like an algebra problem that needs balancing? Can you tell me a little about your approach?
Wow, that's very flattering, thank you.
First off, I have to give most credit to David Hobby at strobist.com for putting together an excellent tutorial, lighting 101. So I'm pretty sure no matter what I type, he has already typed and it is probably better and more understandable. If you have not gone through the Lighting 101 course, do so immediately. It doesn't take long at all, maybe 5 or 10 hours if you actually do the exercises.
David starts off by coming to terms with "flash" light and why it seems complex. It's so fast, it's hard for us to get a grip on it. However, a trick he uses is to simply pretend that the flash is always on. Pretend it's a flashlight basically. When you start thinking like this, you will see that setting up the position, direction, modifiers, power ratios, and gels is MUCH more intuitive.
Another trick is to understand where the shadows are going to be. This is easy. Simple hold the flash up to your eye. Everything you can see, the light will hit. Everything you cannot see, the light will not hit.
How do I think of light? Hmmmm.... first off, as mentioned before, I do not get into the math of it. I had enough math in college to make my brain bleed, so it's not that I'm incapable. I just love the paradox that even though light is predictable, it still does things that we sometimes don't expect. That's exciting to me. I don't do guide numbers or metering even though I have the formula to determine proper flash power taped to each of my LP160's just in case. I don't like making it into a science. I like keeping it an art.
Part of it is not thinking about the light so much as I'm thinking about the subject. How do I want the subject to look? Serious? Beautiful? Mysterious? Edgy? Suppose I want someone to look like they mean business. I literally create a vision in my head of the MOST serious image that I can think of of that person. Once I do that, I look at that image in my head and I figure out how my lights are setup.

I know that sounds crazy but really, light is light, no matter where it comes from. At this point I also determine focal length, background, and composition all from the vision that I have created.
Here is a shot that I pre-conceived... It didn't work exactly as I had planned but that's why I love flash photography. It is both a challenge and often unpredictable (at least as far as our minds can forsee).
Don't mess with Ricky by
Tim Herschbach, on Flickr
If I imagine an awsome shot complete with awsome light, I simply have to deduce that light. This is the part that comes with experience, but seriously, with a little practice, it becomes quite easy to look at anyone's photo (unless it has been heavily processed or modified in some way) and get VERY close to the fundamental setup they used without them saying. Obviously you can't determine the model of the light they're using or maybe you can tell if that fill light is from a reflector or a bounced flash, but it usually doesn't matter. It's the QUALITY and RELATIVE STRENGTH and COLOR and DIRECTION of the light that is important in the end product. Not what is producing the light.
Now, I do not, and currently cannot pre-conceive every lit shot I do. Sometimes I have to setup a shot that someone else wants and do it quickly. This quick & dirty method is fun too because you only have a vague idea of how the image will finally turn out when you click the shutter. But it's also stressful when the setup you have just isn't working. I've found the best solution for me is to scrap what I'm trying to do immediately and try a completely different setup (different light positions, different ratios, different background, etc...)
Here is one that was not at all preconceived but was setup and shot in about 10 minutes. I wanted to take her portrait but had no idea what I wanted to do. She suggested in front of the Texas flag which was on flag pole in the corner. I stretched out the flag for a backdrop, put an "up" light on it, but an umbrella on her, and reflector on the left, and a hair light in back. It's a setup that's basic, works on just about anything, and is very effective. (Composition is a little wonky, but I can live with that.)
Lauren Freeman, Texas Proud by
Tim Herschbach, on Flickr
Finally, I want to stress my work flow. It is not nearly as easy or intuitive as the photos I have on my photostream might suggest. It's time consuming and I take a LOT of test shots, adjusting my power levels and what have you many times. For the shot immediately above, I probably took 5-15 test shots to adjust lights. I have taken more for some setups. Occasionally the first test shot will need no adjustment, but that's quite rare. I simply could not work like I do if I was shooting film because A.) I could burn half a roll just on test shots, and B.) Of course I couldn't take "test" shots because I wouldn't be able to see them anyway. I am in AWE of guys that did this kind of stuff with film. I feel like I'm just playing around when I think about what they had to go through.
I want to be as helpful to you (and anyone else) as possible. If you ever want my input on something, feel free to send me a PM or "Like" a post to draw my attention back to it. I would be more than happy to work with you one-on-one if you'd like. I love this hobby and I love helping others become more proficient where I can.