A suggestion, from my guide book, it that once you've attempted a few contour drawings of individual objects then it's useful to move onto groups of objects - particularly if they're overlapping and parts of the scene are hidden. The reason for this is that such arrangements teach you how to handle the space between items and become more comfortable with the elements which are hidden (even though you know that they're there).
So today I poked around in my garage for some tools with strong enough features to be interesting when reduced to contours and yet weren't so complicated that I wouldn't be able to draw them. Thus I emerged with a wrench and some, what I can only describe as, hand shears (although I'm sure that they have a more technical name) and posed them as shown. The key here was to identify significant shapes, such as ellipses and rectangles, and to use these as indicators of the plane of the tool.
Looking at the result I can see some positive features with the wrench appearing to be lying on the paper while the shears are more vertical. Also the rivets don't appear distorted and details such as the spring and tension-screw come across nicely. Unfortunately the shears feel a little under-cooked and the teeth of the wrench are too deep; this is a great example of getting carried away with the drawing rather than looking at the scene to see exactly how wide the jaws were. Now it's easy to notice the negative space between the jaws but I sure didn't at the time:
For comparison this is how the tools were set up and while flicking back and forth between these images I sensed general agreement in arrangement and orientation of the various parts. Certainly the wrench looks a bit porky but even so in my eyes it doesn't feel too unconvincing:
