Drive slower

I get 16..sometimes 17 with my 2001 5.4 Supercrew with towing package on
highway driving and I consider this to be good mileage for this type of
vehicle, I doubt you can improve it much more than that. Maybe try driving
it a little more gently and see if it helps but in my experience this
doesn't make much difference with this truck. Towing a 24 foot travel
trailer I get 10-11 on long trips , usually with A/C on and a full load of
fuel and gear, again I consider this to be pretty good and I don't expect to
improve it much.

If you have running boards or anything similar, get rid of them
Large mirros make a noticeable difference. I have noticed people on the
interstates lately with one or both foled up against the body.
Tonneau cover, or even better, a bedcap.
Front air dam

All of these will only help highway fuel consumption. The total of all of these
might be 1 mpg
Around town, the biggest factor is vehicle weight, which you can't do much
about.
High tire inflation pressures do help some
Texas Parts Guy

How many miles you got on that?
Try the other suggestions...plus...try a different motor oil. Several
manufacturers (eg Quaker State) offer special blends for high mileage
engines and/or an oil additive such as Restore can't hurt.
My 2 cents.
Cheers.
Joe

P.S. I've recently purchased a can of Restore, but haven't had the chance to
try it yet.

Get a performance air filter (K&N, or my favorite, S&B, the latter available
for $40 at www.stylin.com), Switch to full synthetic motor oil, install dual
exhaust (if you do this, make sure it is aluminized or stainless steel.  Your
truck comes from the factory with an aluminized system.  Regular steel pipes
will rust out QUICKLY, from the INSIDE.  some muffler shops will paint the
outside of regular steel pipes, but in doing this, your entire system,
including mufflers, WILL STILL RUST OUT from the INSIDE. If ANY of the
exhaust is bare, untreated steel, mild steel, it WILL rust out the aluminzed,
and even STAINLESS parts FAST.  If you go all-out and get headers as well,
make sure they are aluminized or stainless.  Most are only painted on the
OUTSIDE.  Remember, ALUMINIIZED OR STAINLESS.  I can not stress this enough.
Otherwise, you are wasting your money. As far as mufflers go, Flowmasters are
too pricey.  Get a single 2 1/2 in. inlet/dual outlet Hooker AERO Chamber
muffler.  The one for the 5.4L V8 F-150 costs about $65 (www.jegs.com or
www.summitracing.com) The AERO Chamber Mufflers are about $30-$40 CHEAPER (A
PIECE) than the Flows, will last longer, and will actually work BETTER.
(Flowmasters are good mufflers, but there are better ones out there),
Platinum spark plugs help, as do performance plug wires, Throttle body
spacers, especially ones like Poweraid that have a helix to make the air
'spiral' help out, hi-flow Mass Air Sensors help, hi-flow catalytic
convertors are good, (just make sure the ones you purchase are emissions
compatable for your area), a toneau cover or a flow-through tailgate helps
alot , an electric radiator fan as opposed to a mechanical one help out alot
too, taller tires help, however if they are TOO big, and they aren't TOO
heavy, your speedometer will need to be recalibrated (don't worry, it's not
hard to do), a new O2 sensor helps, a new breather element or a new PCV valve
is will help a little, and if you have a lot of these modifications combined,
a new computer programmer will enhance it even more.  Just be sure that you
don't go Rev-happy, or you will defeat the purpose.  ALL of these
modifications will also increase horsepower and torque, as well as gas
mileage, especially when tuned right (hense how the new computer programmer
helps).  some of these mods are on the less-expensive side, some aren't.
Have fun!