First step is use the search feature, mileage is a commin topic. Things to improve,
tire pressure is #1
stop long idle times, no need to warm up a diesel for several minutes
Change the intake to one that flows more air, or at least do the zoodad mod and remove the stock muffler and either straight pipe it or install a walker BTM muffler. Letting the 7.3 breathe makes a huge difference!!
Make sure you habd calc all mileage, don't rely on the lie o meter.
As for what you should be getting all depends on what gearing you have? If it is SRW, you should be in the 16-19 mpg range, depending on your skinny pedal activity.
For best mileage keep rpms and boost as low as possible, anything about 2000 rpm's kills mileage.
Remember we are now on Winter fuel in most of the country, my truck drops 2-3 mpg on winter fuel.