Your bill depends on many different factors.  Some of the main ones are:

• Temperature:  If you heat with gas, it goes without saying that cold weather calls for more usage.  The gas is used, we read meters for billing, then you are billed, so it is possible the bill you have could be for usage during the prior month.  This is why you may have what seems like a higher bill in the Spring when it is warming up in March and April.  You are still paying for winter usage.
• Thermostat:  The higher you set your thermostat on your heat, the more it will come on to maintain that temperature.  You probably won't hear it come on a lot if it is nice outside, but when the temperature drops at night while you sleep, your heat is most likely running.  If it is on 65 and it is 50 outside, your heat will run.  Try to keep it 68 or lower in the winter if possible.
• Appliances:  The more you have using gas, the more gas you use.  Also, older appliances are not nearly as efficient as newer ones.  Take logs for example...most logs in the past were "vented", meaning the chimney damper had to be open to allow fumes to escape.  Since heat rises, where does the heat go?  Up the chimney.  Newer sets are "unvented" so you can close the damper and get more of the heat from the logs.  Tankless water heaters only run when you turn on the hot water, so you are not constantly reheating 40, 50, or more gallons of water around the clock.
• Commodity Cost:  Natural gas is bought for resale on futures contracts, meaning most of the gas you are using today was probably bought months ago.  We do this to lock in lower rates.  It is all about forecasting prices, temperatures, and how much gas we think will be needed - which is never an exact science.  While we may be able to purchase gas for $2.00 per dekatherm on a futures contract today, if we wait until the actual time it is needed, it may cost $20, $30, or more.  We make every effort to hold rates as low as possible and are generally competitive with other gas systems around the Southeast.
• Insulation:  Older, draftier buildings will use more energy to heat.  The more insulation you have and the more you can seal up leaks, the less gas you will use.