Autodesk Sketchbook is a fantastic piece of software which helps you to create superb sketches easily and quickly.
You can use it with an ipad / tablet as well as a graphics tablet. Equally if you have a touch screen laptop and a stylus you can use it with this software.
Here's the handout which you can print to go with it. It's a simple tracing activity which is designed to help you get a feel for how to generate super clean line work. It makes for a great warm up task.
Here are some simple exercises which get gradually more difficult and are to get your brain in to 3D mode.
This hand out takes you through some simple isometric cuboids which will help you to establish an understanding of basic Isometric.
This hand out gives you activities in which you need to translate a projection drawing into an isometric shape and then vice versa.
This is a step by step guide which shows how to sketch the speaker unit you traced (above) in isometric using a method called crating.
Crating is where you start off with a block shape in isometric an gradually cut away or add to it until you are left with the shape of the product.