I have had a few things that have helped my MPG.

Actually just today I took off my air dam, we will see if I get any increases there.

But last year I followed a process using seafoam to clean out my engine.
I dont know it the exact steps but you can find it some where on here.
but you put some seafoam in your a full gas tank along with in your oil. then you have a process where you pour some in a vacuum line.
I gained about 2-3 mpg after this but only lasted for about 6 months if that. I am going to put some seafoam in my next gas tank to hopefully clean out some things and gain something back

Also I recently turned the distributor so that the idle actually increases slightly. There was just one bolt that I had to loosen and it rotated easily. This helped it run more efficiently. Be sure to mark its original position with a sharpie incase you need to go back to the original setting due to a decrease of mileage.
This is something that you can move a few times until you find where it works best. I gained about 1-2 mpg after this adjustment.

I have a 2001 xterra se 4WD. I average 14.5 mpg and have seen as low as 10 while speeding and was actually only getting 12 when I bought the car w/ 77k miles on it. The seafoam treatment jumped me up to 15 for some months.
Currently 116k miles averaging 14.5

there is also the idea of cutting weight, of course there are a number of unique ways to do this.

overall we all know that xterras dont budge must here