When I was in art school I took a fabric design class and one of the first assignments was to come back to the next class with fifty designs. Believe me there was a lot of moaning and groaning about coming up with that many designs. Then the professor explained that he didn't want full blown designs, just thumbnail sketches. So off I went and did a bunch of drawings, which were really, just doodles and, low and behold, some good things came out of it.
The interesting thing about doing fifty designs is once you have done about twenty, you've considered most of the predictable possibilities. Then the fun begins. You start to think of new ways to look at your idea, different materials you hadn't first considered come into your brain. The first twenty are like putting your car in first gear, by the time you hit 45 you're speeding down the right side of your brain!
I do this exercise nearly every time I want to try out a new idea. I struggle in my head with where to begin and then a light goes on that says, "Make 50 designs!" So the struggle ends I get sketching and at the end I have many possible ways to take the idea. Interestingly when you have so many choices you can clearly see the best because they stand out. I did this exercise yesterday, my first day back really working and it energized me. I have included some pages of my sketchbook from yesterday as well as other times I have used this technique.
So, if you are struggling with where to begin, make fifty designs!