When you draw, it might help to write down exactly what you're thinking. Because if you try to remember something while drawing, they're going to hinder each other and thus you get a blurry image in your head and a meh drawing. (Sometimes I actually write on my drawings to keep something in mind when I get to it. And this is also why I love doing requests, you get a set of instructions and I can keep looking at the instructions whenever I'm stuck or unsure.)
Next like PossumDeMort said, breaking down images definitely helps. And when you start drawing (Even from your head), do this. Forget the lighting, forget the shadows, just put down boxes and relative shapes that symbolizes what you will do. Try to lay down as much of this blue print as possible because when you mess up it will only take you five or six minutes to fix it. As opposed to drawing something, finalizing it, then immediately realizing, you spent an hour on something and you have to erase it. As you get better you can start jumping over these steps because you almost know where things should go.
Which brings up another tip. When drawing, if you're not working on a drafting table, stand up and look down at your drawing from time to time. (Or walk away and come back to it.) Sometimes you will realize some mistakes when you do so. This is why in speed paint videos you'll see artists flip their images, it helps to see mistakes they may have not noticed.
Well when it comes to avoiding making a drawing a mess. Try using a darker/softer pencil (Like 6B) I know it may at first seem like the exact OPPOSITE of what you want, but there is a method to my madness. When you use a 6H it dents the paper and the eraser can't reach it and so over time it builds up and you can't erase anything, instead a softer pencil will not make a deep mark and is instead easier to erase and see what you're drawing. Then why do 6H pencils exist you ask? I dunno
. Also If you're left handed, start drawings on the right, and vice versa. This way your hand/arm doesn't smudge stuff. (So let's say I start a drawing on the right and I'm right handed. When I move over to the left side, I smudge, everything on the right with my arm and it takes time to repair the damage that I've done.) (Well unless you're working digital where nothing smudges.)