I have an 04 f150 supercrew. My best has been 24.7 hwy (today - see thread in introducing yourself). I dont do much city driving at all, but do have a stretch of bypass highway that is getting cluttered with stoplights. Im at about 20mpg there. The SGII is a great help when it comes to driving optimally. I have made a few mods that appear to be working out as well. I have a few more things to try and improve more.
Of course, the recommendation is to find a fuel-efficient vehicle to augment commuting, and have the truck to haul when you need to. I generally say to run the calcs -- number of miles driven vs. money spent on fuel to see if a small car can be added.
If that's not an option, this site is here to help. I think jeffreymccoy has a great aero start and good advice with the SGII. Driving style can go a long way too.
Reminder to folks with large vehicles -- every little bit helps. Going from 15 mpg to 20 mpg is a 33.3% increase. For someone with a 50 mpg vehicle, that translates to 66.7 mpg, or nearly 17 additional miles per gal.