You have a V6 Mustang. You are pretty much set up to get the best MPG possible.

What type of numbers are you looking for out of it? Keeping your filters clean, your oil changed, staying out of city traffic, and going light on the gas pedal are all ways to get the best possible MPG. And keeping your tires inflated as @yaser mentioned as well. 

Mustangs aren't made for excellent MPG, even the V6 models. If you have done the above and still aren't happy with the mileage, you might be in the wrong car for your needs. 

Aka, drive like a stereotypical ‘grandma’..

I think you maybe have bought the wrong car, if you care so much about gas mileage.

Besides the above suggestionts, make sure your tires are inflated properly. 

Don't drive your car like you stole it, avoid stop and go city traffic, go 65 on the highway, and avoid jackrabbit starts and stops. Basically, don't drive your car like it's a Mustang. Coast out red lights as you approach them if you can and try to time it so you will still be moving when you get to it. It takes a lot of energy to accelerate a 3800 pound car from a stop. It takes significantly less to maintain the speed, or close to it. Also limit your use of the brakes. Every time you slow down with your brakes, not via rolling friction, you're wasting energy from the gas vs rolling out the need to slow down. Don't worry about this if it's unsafe to do so. 

The biggest one is don't idle your engine. You waste a lot of gas idling for long periods of time. Something like a quart of fuel per hour if I remember right. 

Drive less.

You bought a muscle car.