When I drive on the highway, this is my rule: I usually park the speedo at around 55 (of course allowing for decreases going up a hill/increases going back down), until a slow-enough big rig passes me and gets in front of me in the right lane. I think of this as "fishing"... just waiting at 55 miles per hour for a suitable truck to pass me. If they're going 65 or less, I figure the benefit from drafting will outweigh the negative effect of driving at the increased speed (which, after all, comes from increased wind resistance that one would presumably be negating by drafting). So once the big rig passes me (not "flies by me" - then it wouldn't be a suitable target), I give 'er a little assist and slip into the wake envelope of the truck. I stay about 1.5 car lengths back. My mpgs invariably shoot up!

I've only recently started doing this, and it made a huge difference in my highway mileage (see my last few tanks, which admittedly aren't that impressive by true hypermiler standards). I can't wait for the summer to come back around, so I can really apply my newfound knowledge of hypermiling techniques and maybe get tanks in the 60s more often.