Sometimes after some time has passed you look at your work and see those mistakes that you did not notice at first. And then (well, at least me) you scratch you head and puzzle over the fact that you did not see that very apparent mistake in your work.
Why is that? Well, mostly it’s because your eyes get used to staring at one thing and after you do take a break your eyes approach the same subject with a fresh look, which means that you start to notice things that you might have not seen previously. This is why taking breaks while you work is a great idea. I take a ten minute break every hour or so, so that I can stay concentrated and not rush things (I do tend to start rushing after about an hour into the painting) and so that my eyes get a short break and I can re-start work with a fresh look on things.
Another way to notice those little pesky mistakes is to walk away from your artwork. I work upright so it’s pretty easy to do. If you are working in a sketchbook or on paper that is lying flat, a good idea is to get a drawing board and then tilt it against a wall or a piece of furniture and walk away from your work. Even a couple of steps will switch up the view for your eyes and you will start noticing things you might have not noticed before.
Using a mirror or turning your work upside down is another way. This works amazingly well when something in your drawing has its axis off centre or if you are struggling to get the likeness.
A camera on your smartphone is another thing you can use to see what needs improving. First of all the display is smaller than the size of your work (unless you are working on a mini painting or drawing) and seeing your work on a different scale will trick your eyes and make them think you are looking at something new.