Best add-ons or mods to boost mileage? 98, 5.7 Vortec

I have a 98 K1500 Silverado, 5.7 Vortec. It is in great tune and runs smooth as silk, but mileage has dropped considerably. I bought it two years ago with 75,000 miles, now has 90,000. Highway mileage used to be around 18, now around 13. All tune-up items are new and I just ordered two brand new GM O2 sensors.

Hopefully, the sensors will help bring mileage back up to where it was, but I am curious about other mileage-boosting mods. I don't care about power levels, it's got plenty of power. Is it wise to get a new chip or re-program computer somehow? How about headers? Again, forget power, has a set of headers helped anyone with mileage? How about any of those throttle body spacers? Any other ideas?

Thanks

TB spacers work for the TBI engines (87-95), but I dont know about the newer multi-port FI Vortecs.

I dont know where you live, but cold weather really hurts gas mileage, so that coudl aslo cuase a drop off in mileage.

A Flowmaster muffler seemed to help my truck (92 K1500, 350 TBI, 130,000 miles-averages 18-19 mpg mixed driving). I also have an AIRAID air intake, but I can't reallly say for sure if its helped or not (cold weather-I always let my truck run for more then 5 minutes when its cold befroe I ever put it in gear).

I have a 97 with some stuff(read below) Definitely didnt hurt. I have about 113,000 miles and in the winter with about 70% city 30% highway I average about 13-14 mpg. In the summer it is about the same but maybe a little better. On highway w/ the ac on I will get over 18. Like chevytrucks92 said, depending on where you live(how cold) does make a difference and I am like him when it comes to letting my truck idle for a while when it is cold. I figure if you want great mileage buy a 4 banger.

get a K&N filter it will help. Also open the exhaust up. Flowmaster or something like that. Let that baby breathe!

I had a 96 sierra a few years back with the 5.7 and when i bought it used it already had a k&n filter so i did not have to buy it. I bought larger tires for it, the truck came from the factory with 245/75/16's and i went to 255/85/16's and there was a considerable drop in performance. The 245's were 29" tall and the 255's were 33" tall. Before the tire change when going approx 30km/h and if i floored it, when it kicked down it would break them loose and then chirp them into second. It no longer did that after the tire change.........UNTILL i had dual exhaust installed. I gained back the lost hp because of the oversized tires if not even a bit more. Wish i could say the same for my 2000 silverado with the 4.8........