Here is my thumbnail creation technique. The basic philosophy is that you can mold random shapes to form a more distinct shape to use for character development or other concept art projects. I.e. I also use this shape technique to come up with vehicle designs or prop designs, like weapons or a futuristic lunch box.
Silhouettes are great for concept art because they don't restrict you to conform to a specific design; in my honest opinion, I believe they lend even more to letting your imagination flow. Also it is great practice to see things in shapes and form and not the little details that make up inside.
#1: Let your mind loose. It doesn't matter what shape you make. Random shapes add up to making up new shapes in later steps. Scribbles are great. The more random or unidentifiable the shapes are, the better!
#2: Here are a collection of the random shapes that I have made. Know that this isn't our final thumbnails page (in fact, there isn't really a final one. The more pages, the merrier!). They are unidentifiable in terms of what kind of character this is going to be. We can tell that it will have two legs, but is it female? Male? A creature of some sort? We'll never know...
#3: Putting them together. This is where it gets kind of technical. I took a shape (a) I liked from the previous canvas and pasted it on a new one. I copied another one (b) as well and pasted that onto its own layer.
Shape (b)'s layer should be on multiply. This will make it darker, but if you use the level option (cntrl+L) and move the right hand arrow to the left, it will make it "transparent".
Hint: The multiply blending mode makes all white in that layer transparent.
Duplicate, transform, flip horizontal and put it all together. Do whatever your heart desires. The main point of this step is the multiply blending option and how it opens brand new shapes for you.
#4: Character ideation. Repeating step 3 numerous times has let me form brand new shapes from previous canvas. In here I was focused more into narrowing down my direction but yet still did not restrict myself from what it might be like. Also, know that you are never restricted to do anything on this phase of your work flow. Anything goes - warp tool, painting on top of the shapes some more, customized brushes, etc. etc.