Raise tire pressure
I kinda doubt that any motor type add ons(tornadoes,CAI, aftermarket exhausts) can improve mpg.Now I suspect that one of those piggyback ECM modules might help if you could get one tuned to mpg rather than power.I wouldn't expect an awfull lot from one of them since the manufacturers are really trying to get good mpg already.
Is your truck one of those with the really high pressure truck tires-like a 3/4 ton? Some of them run 80 psi already.If you truck calls for normal sort of pressures-35-40 psi) you can jack the pressure up to the max on the sidewall.It will improve your mpg a little, but the ride will degrade a bit-harsh.
In city driving ,I think accelerating fairly briskly to about 5 mph above the limit,and then completely lifting until you glide to about 5 mph below the limit gets better mpg than the usual 'drive like you have an egg under your foot."If you know your route pretty well, you can judge when to lift early and glide up to the redlight you know you won't make.
If you are first in line at a redlight the above is no problem; if you aren't first,then you have to pause a bit to let the first guy get a bit of a lead on you,so you don't run him down at the 5 mph above the limit.
Luck,
Charlie