When we first moved into this house about 15 years ago, I was astonished at how high our natural gas bill was. We use gas for our water heater and for heating the house in spring and fall (when running the wood stove would make the house feel like the tropics). I was always careful to wash most clothes in cold water, used an efficient dish washer, make sure our filters were clean, and turn down the high temp on our water heater. Yet our gas bill was still $300 or more every month.
Over the years, we avoided using gas heat as much as possible, and were able to lower our bill to $20 - 25 a month...unless it was spring or fall, when gas heat was our only comfortable option, and our bill went back to hundreds of dollars.
Then our house became infested with mice. I could watch them coming and going through the heating vents throughout our house. Ugh! Once we got rid of the mice, we knew it wouldn't be safe to run our gas heat without having the ducts cleaned. Mice and other rodents can have serious - even deadly - diseases in their feces and dried urine. And you don't have to touch the dust of their urine or their feces; just breathing it in can make you very, very sick. So I hired some pros to come in and clean the ducts.
Funny thing was, they discovered that every single duct in our house was no longer attached to the vents! So for years, we'd been mostly heating the space underneath our house; very little heat was actually getting in the house itself. It took the workmen a handful of minutes to reattatch the ducts to the vents...and guess what? Ever since, our gas bill has been very reasonable - even in the spring and summer. We never get a bill over $30, and around $25 is much more common.
So if your gas bill seems high, and you've tried all the usual advice about lowering it, you might consider checking to see if all your ducts are properly attached to the vents!