One of the great struggles in drawing and painting, for me at least, is the conceptual mind interfering with what I see in front of me. Instead of painting what I see I end up painting what I think I see, not what is really there. A face may become more stylized or a shadow lose its uniqueness. Maybe this is good for some art if you want to express yourself or part of your personality, but what I find beautiful in a painting is to see the light and form of a moment interpreted in paint. I'm still learning so half of my painting turn out awful and the thing I paricularly don't like is when they look amateurish and cliched, this is normally a result of me going on autopilot and painting what I thought was there instead of what really was there.
The point is to see your painting as it really is and this can be done in various ways. Looking in the mirror is a great one. Sometimes I'm amazed at how odd my painting looks when held in a mirror: massive jaw, wonky eyes, it's embarrassing. A hand held mirror can be useful to see both the picture and the subject at the same time. A computer can be used to flip the image or simplify it in various ways: if you are having trouble seeing the tones you can increase the contrast until it's almost black and white, or you can apply a filter to try to get a different view of the picture, with limited layers for example. You can even apply a filter to imitate the image in paint and then paint the computer's painting. I used to do this more but to be honest the more you paint the easier disctinctions are to see, plus there is something to be said for a human's interptetation. Now I like to get everything in the right place and I let my eye interpret the hues and tones. Everyone seems to use Photoshop but I found the completely free PhotoScape was fantastic and the things you can do with the hues can help with any electrically lit photos you have that may look yellow. Play with the sliders and curves too much though and you will end up with something that looks very artificial, use with caution but a little boost on the contrast never hurt anybody.
Another useful trick if you are working from a photograph is to look at your painting and the photo upside down. This can help you to see the shapes you are working from rather than what your mind thinks they should look like: it can be easy to make the eyes too oval or the whites of the eyes too white. The painting of my dad I posted earlier I painted completely upside down at first. When I flipped it I had got some of the features a bit wrong (the size of the forehead made him look a but like Homer Simpson) but it had captured something special about my dad. It's a kind of magic to project somebody's character through the careful placement of colour and form.
The final tool I can think of for the moment, and the subject of the post, is the very useful observation of negative space in viewing a subject. Look at the spaces between the shapes as much as you look at the shapes and things will slot together better. It's a meditation on the Taoist duality of fullness and emptiness, existence and non-existence, but not really, I'm just being pretentious. It is actually just trying to see what is really there.