My favorite way for getting a custom white balance is using my ExpoDisc.
I have found if the subject is facing me and the light is from the side, I face the camera with the ExpoDisc on it so it is pointing towards the camera position. The figure above is to help you understand the concept, but you can modify it.
One way you can modify it is as long as the light is the same where you are standing, then you could cheat and take a reading from where you are. The problem that can arise is if they are lit by Window light and the camera position is in the shade then your color balance will be off if you do not take it from the subject's perspective.
Use the wrong color sometimes
Yes, I just said to not use the proper color sometimes.
Most all Hollywood movies that show night time scenes are often shot during daytime. How do they achieve that look? Set the camera to incandescent which will give you a blue cast making everything look like it is lit by moon light. Next underexpose the scene. I find this is where a spotlight on the subject and underexposing the rest of the scene can help you set the mood for a night scene.
High Tech Look
If you have daylight in the scene and you light the subject with bright incandescent light and set the camera to incandescent then the subject with be the correct skin color and the area lit by daylight will be blue.
Under florescent lights if you have the camera set to incandescent they will turn blue as well, just a different blue than with daylight. If you light the subject with incandescent you get that blue affect.
CSI Miami uses the technique of letting the fluorescent light go blue by lighting the cast with incandescent and setting the camera to white balance for the incandescent. This way everything lit by the fluorescent goes blue while skin tones look natural.
You can fake a sunset by putting a CTO filter over the camera lens making the scene look orange. Then you can use a flash and put a CTB filter over the flash which puts out a blue light. The subject looks correct with the skin tones but the rest of the scene is orange like a sunset.
When you are not sure what you are going for and you just let the camera do it all, this is the surest way to have the color of your photos announce you are an amateur. Want to take your game up to the next level, learn how to get correct skin tones and when to go for an effect.
Why so many photographers choose to shoot Black and White
One of the biggest signs for pros who don't know how to get good skin tones is to go to black and white. This is the easiest way to eliminate the sign they’re still an amateur when it comes to color balance.
This is why I think so many wedding photographers shoot black and white. I think they are not using it so much for an effect or creating a mood, but they don't know how to get the color correct. Most likely they shot everything in JPEG and if you are off with the color in JPEG correcting it in post production is very difficult as compared to working with a RAW image.