Rather than drawing my ideas in 2D, I use polymer clay as a sketching medium for designing my metal clay work. It means I can easily work out not only the actual look of a piece but the 'engineering' as well. This saves the worry of the clay drying out or the feeling that once you have invested time, energy and money into something you feel you have to keep going even if it isn't exactly right.
With the polymer I feel much freer to experiment, I can see how a piece works in 3D and I can repeat the process until I'm really happy with it. Then it's much easier to 'translate' into metal clay working quickly and fluently.