There are a few things you can do, but don't expect much. As others have said, the best way to better fuel mileage is to carefully watch your driving habits. I have a LOT of friends who are always 'pumping' the accelerator, that is, the give it a jab, then let up, then give it a jab, and so one, until you're car sick. That's while they are cruising down the freeway in no traffic. I imagine their Toyotas getting about 15 MPG!! Keep a steady pressure on the throttle when cruising, or better yet, if you live in a corner of the world where the ground is fairly flat, use the cruise control when ever you can.
Keeping your cruise speed down can also help. That can be dangerous in some parts of the US, though. But you can keep it a little lower and it will help.
Adding a few lbs. to the tire pressure and watching it like a hawk is also helpful. Tire pressure will rapidly change with temperature, so check it any time the weather gets cold.
You have a '04 Tahoe? I think that's the same or similar front end that we have on our GMC Sierra (similar shape, anyway). On that truck, I added a 06 (I think that's the year) Tahoe chin spoiler. I had to drill small holes and used a series of small stainless bolts and lock nuts to secure it, but adding the spoiler netted us 1 MPG improvement. This is a diesel, so YMMV, but it should help some.
Lower the truck a couple of inches. The less air under it, the less air that can drag you down. Lowering it some will help the spoiler, too.
Find the lightest tire and wheel combination you can get. I don't know if the Tahoe/Yukon hybrid wheels fit that truck, but I'm told they're particularly light. You can probably find take-offs for a decent price.
Get all the unnecessary weight out of the truck. If you have a bunch of stuff in the back, get it out. The less weight you have to carry, the better your mileage.
HTH.