From UX Matters:

Here are a few things to remember when using Blend:

1. Don’t get hung up trying to create reusable, production code while designing. Doing this is detrimental to your exploring different possibilities. Instead, simulate behaviors through animation to get your intent across.
2. Don’t give up pen and paper. I still do a lot of sketching—either with pen and paper or on a whiteboard—before going to Sketchflow. Even when using Blend, I always keep my sketchbook within reach. Sketching helps me work through design issues.
3. Don’t get bogged down in details—especially visual design—when you are exploring different design options and possibilities through sketching. Dive down into the details and the visual design, once you are ready to prototype.

Remember, your tools should not drive your design process. First, focus on the design problem at hand, then decide which tools you should use to solve it.