To expand about a few things, let me add the following:
1. Indeed, you can really kill your gas mileage by stomping on it as you start from stop signs and stop lights, and stomping on it as you move through traffic. You really can, most of the time, just take your time, and the jackrabbits will go around you. In fact, make it easy for them to do so, if you can. And no worries -- it's very likely they won't get more than a block or so ahead of you, for all of their efforts. Meanwhile, you can just relax, take it easy, get great gas mileage, and coast up right behind them at the next light.
2. Speed on the highway can really make a difference, too -- this is a good time to keep your eye on the Average Mileage display. If you take it easy, perhaps even keeping it down to the speed limit, you'll probably see the Average Mileage start going up. But if you're one of the fastest cars on the freeway, it's likely that you'll see that Average Mileage number start going downhill. Once again, just take it easy, keep it around the speed limit, cruise along in the slow lane to whatever extent you can, relaxing and enjoying the drive, while the idiots in the fast lane are punching it and then stomping on the brakes, weaving around the different lanes, and working their tails off. Again, no worries -- it's very quite likely that you'll coast up right behind some of these guys when you get off at your exit.
3. Finally, you really can't say you're getting good or bad gas mileage if you're not calculating it properly. You really do need to keep on going until your "low fuel" light comes on, and then go ahead and refuel. Make sure to get receipts for your gas purchases, and write down the car's mileage, the Trip A mileage, the Average MPG number, and the Cruising Range number. Then, reset the Trip A and the Average MPG display after each fillup, so they both record your results for each tank of gas. By all means, keep track of your mileage, either on a spreadsheet, an app for your phone, or a web site like Fuelly:
http://www.fuelly.com/
Just go ahead and establish an account, and get moving. Though I'll give you a tip that I sure prefer with the site -- don't have it figure the miles for you based upon the odometer, because that just makes some presumptions you might not want to make -- just use the number of miles for each tank, based upon your Trip A number. That sure makes the site work more like you might expect it to -- I sure wound up preferring to work with the site that way.
At any rate, bottom line, you need to record your mileage over "full tanks of gas," not little top-offs. Little top-offs can record "nothing but in-town driving" or "nothing but freeway driving," or anything in between, and really won't record your true average over all kinds of driving.
Apologies if that's all old news to you, but by all means, if that'll help you get better gas mileage, go out and get better gas mileage!