If at first you don't succeed, try, try again. You wouldn't know it, but this painting started out as a baaaaddd abstract painting. Apparently it was so bad that I didn't even take a photo of it. So, having decided it was going nowhere, I started painting this new painting on top:
The good thing about painting over an old painting is that a) your fear of messing up the canvas is gone; and b) you already have a body of paint on there so your new painting has a nice richness to it.
A couple things to note when over painting:
1) While you CAN paint oils over acrylics, you NEVER paint acrylics over oils. Think of it like this: what happens when you mix water and oil? The oil rises to the top. Same with painting, the oil will try to rise to the top, forcing your acrylic paint off (OK, this may only happen over an extended period of time, but if you want your painting to be archival, just say no!).
2) Turn your old painting upside down before you start painting the new one. This helps get rid of the distraction of your first composition and allows you to concentrate on the new one.
By now you're probably wondering why I take in-process photos of my work. And no, it's not for these mini-tutorials (although they are helpful). Rather, I had a professor who suggested we take photos so that we could mull over the painting while not in the studio. They don't have to be good photos (in fact, this is a very dark shot of the painting), just quick shots even on your phone so that you can see it.
This can also be helpful if you paint over something you like and want to bring it back.
And, I've noticed that looking at your painting on a (camera, phone, or computer) screen changes how you see it. You notice different things, particularly about composition, when it's once removed.
Another good trick is to look at your painting in a mirror -- this means you're seeing it in reverse and it's much easier to see if you've made a terrible perspective mistake or something like that. I use this all the time, I even went out and bought a 5"x6" hand-held mirror from the dollar store expressly for this purpose!
(No, I did not do this painting in only 3 sittings, although it was one of my faster paintings).