As everyone here knows already, probably the simplest way to increase your fuel economy significantly is to reduce your highway cruising speed (NRCan quotes about a 1% increase in consumption for every 1 km/h over 100 km/h).
Despite this fact, I have noted that people sometimes instinctively recoil from the suggestion, citing "going with the flow" as the reason.
I often wonder how much of the objection stems from the practical (ie. actually significantly disrupting traffic flow), and how much is psychological (the stress generated by the inevitable tailgaiters, who inattentively ride your rear bumper for a while, then "wake up" and roar past).
Regardless of the root of the objection, my prescription for people who are sensitive to the issue of slowing down is this:
Enter the freeway at your usual pace, and drive as normal until you pass a transport or other large vehicle in the right lane which is driving slower at a steady pace. Change to the right lane
in front of that vehicle (keeping a respectable gap of course). You are now free to drive at the reduced pace of the vehicle
behind you. You are no longer responsible for blocking faster traffic; it was flowing around this vehicle anyway.
It's sort of "reverse" drafting: they're breaking the flow for you - not of air, but of faster traffic approaching from behind, and you're capitalizing on that to improve your fuel efficiency.