It's easy to get obsessed with trying to find a neutral, but don't lose track of what it is you're trying to do: you're trying to remove a colour cast. That is, you're trying to get rid of a tinge of colour that effects the image, in whole or in part. The way to do that is to find a part of the image that shouldn't have any colour, then make the adjustment that removes whatever colour is actually in that area. If the same adjustment is made across the whole image, the colour cast will be removed entirely. CM does all of this for you, except the first bit: finding the area that shouldn't have colour.
A grey area, by definition, shouldn't have colour, so that's best, but as you say, sometimes there isn't a grey thing in the image. But black and white areas also shouldn't have colour, so they can work too. They're not as effective at making correction curves as grey areas (for reasons Mike will be able to explain) but they do work. In an image with no obvious grey area, I look for a white area and try a neutral point there. If that doesn't work I try a black area. If the effect still doesn't look right I drag the neutral marker around a bit looking for the "sweet spot" - I sometimes drag into random areas of the image to see what happens. Quite often the effect is a good one. If that doesn't work, switch to RGB mode and try placing several neutrals in dark and light areas. This pulls the curves about quite a bit, but often makes a good result. When I get a neutral that clearly is a step in the right direction I hit Apply and then go back into CM for another go. If you can correct 80% of a cast in one crude swipe, getting rid of the rest of it using a more subtle skin tone correction is then easier and more effective.
Finding the right area for your neutral is often more about experience than knowing at a glance what's right. Try a few places and see what happens. If you're really stuck, post the image here.
