How to plan a drawing?
A common practice is to first create a thumbnail. You can do this when drawing from real life or from imagination.

The idea is to get a rough, small, quick concept of what you're going to be working on down on paper. This lets you get the idea out and reference it later as you're working. 

When drawing from imagination, you can utilize multiple thumbnails, then decide on which pose/style you'd like to move forward with. 

An example can be found on Niklas Jannsan's website: Android Arts Doctor Who project. The first image has some thumbnails of a moth-like creature, and then the final image incorporates the concepts laid-down.

When drawing from life or an image reference, a thumbnail is helpful to evaluate proportions. Before starting to draw on the entire page, you can draw a small rectangle in the corner or on another page. If you divide that rectangle up (into 1/4ths or 1/8ths), then draw your thumbnail, you made yourself a handy map and proportion guide. Just be careful that in your thumbnail you didn't exaggerate some proportions too much.

From your thumbnail, you can build up your rough sketches, laying down shapes and forms and building your line-art, but my advice would to be start with the thumbnail and then go on.

Get your idea out, make variations, work from there. Be willing to make changes, and don't enforce perfectionism in your first drafts.

As an aside, a common problem I've seen, and that I face, is knowing that something is "wrong" with a picture, and then erasing it to fix it. However, there's usually not a clear idea of what is making it wrong, or how precisely to fix it. Before you erase to start making changes, you should analyze the part that's wrong to really understand what the problem is. Proportions? Angle? Size? Shape? If you don't know what's broke, you can't fix it.