1. Always stand to paint

There are two good reasons for this.  The first is that you need to keep looking at your painting in its entirety to gauge the whole experience that your viewer is going to see as they walk towards it.  If you are standing you’ll be quite happy to take a few steps backwards and have a look, but if you’re sitting you can’t and you won’t.  Standing back from your painting early on means you’ll catch errors in form before it is too late to correct them.  Standing back later on helps you to tie different areas together (perhaps with colour or repeated motifs).
The second reason for standing to paint is that you have to be prepared to manoeuvre your body around your working surface in order to make the correct mark.  It’s not a portrait specific example, but imagine that you are painting the highlights on some clouds.  The point where you first apply the tip of your brush will be the most lit up and, as you drag your brush away, so the paint will thin out and the light effect will be increasingly subdued.  So if you are painting these highlights on a daytime scene and you need to convince your viewer that the light source (the sun) is out of the top of your painting, you will need to apply the paint with the tip of your brush upwards and then you can pull away downwards towards the bottom of your cloud.  That’s fine, you could do that sitting down, but imagine you were painting an evening scene in which the sun is below the horizon.  Then you would need to hold your brush the other way up (upside down if you like) with the handle above the tip.  To do this you have to reach right up above the painting and hang the brush upside down.  If you are sitting down, and you can’t reach properly, then the chances are you won’t bother.  But if you are standing, you will automatically manoeuvre yourself so that it is comfortable to do so. 

Thomas Gainsborough is famous for extending the handles of his brushes (up to 6ft!) to prevent himself from straying to near to his portraits, and I once heard that another famous artist, whose name escapes me, reputedly ran through the open doorway into another room to decide where the next stroke was going to go and then ran back again.  These examples may be a bit on the bizarre side, but one artist’s trick you do frequently see is that of wearing reading glasses to paint.  If worn just low enough on the nose, it enables an artist to flick their eyes between a detailed view and a distant view without falling into the trap of stepping forwards to apply a detail and then forgetting to step back again.
I know that some very talented artists sit down.  I’ve seen them too.  But I bet you there was once a time when they stood up, which means that when they come to make that mark that isn’t reachable, something in their subconscious, learned in their past, will make them stand up to do it properly.  Also, if you’re doing a fairly small painting you can get away with sitting down as long as you position your chair a good distance from the easel and do not, under any circumstances, allow yourself to move the chair nearer. 

2. Know your studio’s natural lighting conditions

The gentleman painters of old would use their vast inheritances to locate their studios specifically in rooms where the light could only enter through windows to the north.  This meant that direct sunlight never entered the studio and, regardless of the weather conditions outside, the lighting conditions in the studio remained relatively constant.  On a bright sunny day with intermittent clouds the light can be infuriating if you are painting by a south facing window.  It will keep casting patterned shadows of your window frames across your working surface.

If, like me, you fall into the category of ‘beggars can’t be choosers’ try to keep your easel as far away from direct window light as possible or, failing that, hang a thin cotton sheet (I use a sarong which is perfect) over the curtain rail in your studio.  This will go some way to tempering the changes in light without cutting it out altogether. 

For particularly dark days some artists use special daylight light bulbs in their studios because they prefer to work in a slightly cooler, bluish light, which is more akin to that of the natural outdoor light in the northern hemisphere.