As a designer the most important thing is to get your ideas on to paper as quickly as possible. Sketching is therefore a very important skill to master. There are a few things which will help you sketch better.
Draw using your shoulder rather than your wrist.
To sketch quickly learn to sketch from your shoulder. Most people learn to sketch on small pieces of paper, usually no bigger than A4, sketching small images. Because the images are small, people tend to learn how to sketch from the wrist. Drawing a long line tends to consist of lots of small movements from your wrist, as you move your arm along. If you look carefully at these lines they consist of lots of small arches.
Sketching from your shoulder means that you can draw lines in one continuous movement. Instead of drawing by moving your wrist, keep your wrist still and move your entire arm in one long movement from your shoulder. It's not an easy skill to master but once you have, you definitely will see the benefit.
A good way to learn is to practice drawing on large A2 sheets of paper and have the paper vertical. This means that you can't rest your wrist on the page encouraging you to use your shoulder when drawing.
There is a simple exercise that can help. Draw a series of opposing vertical and horizontal dots at each end of a piece of paper. Practice drawing a continuous line from one dot on one side of the paper to the dot on the opposite side of the paper. Gaining confidence in your drawing action is vital to improving your sketching skills.
Choosing the best type of paper
As mentioned above, sketching skills tend to improve if you use large pieces of paper. Learning to sketch on A2 paper is a very good way of gaining confidence in your skill. But most people find A2 an inconvenient size for project work. If you can, try to do sketches for your project on A3. Buy a pad of layout paper. This type of paper is useful because it is slightly opaque. When drawing final ideas you can quickly sketch your basic idea getting proportions correct and then use another sheet of layout paper placed above to trace the original sketch without mistakes and guide lines.
Sketching in pen
When sketching, pencils are the obvious choice, but I recommend using a biro for the simple reason that you can't rub out biro. This means that you will learn to put down only the marks that you really want to. Also you won't be tempted to rub out ideas. Never rub out an idea it shows the examiner that you have thought about your design and not just developed your only idea. Besides, occasionally you find that you can use some element of discounted designs..
Drawing box guides
A simple way to get the proportions of your idea correct is to sketch boxes in perspective. See the 'Drawing in 3D' section for more information about the various 3D sketching skills you can learn to master.