Drafting behind trucks (when cycling) could be a great energy saver. I came across this idea when reading up a blog post by a cyclist.
The idea is similar to birds flying in a V-shape to get a greater lift. While drafting, the goal is to reduce the drag force.
Pacelines in cycling races are formed precisely for this reason. This page has a short summary of what drafting is all about.
Should be trying this out, the next time around.