One of the best ways to keep your speed steady is to know what it coming up. Look ahead of you. Do not just focus on the car immediately in front of you. If you watch the road ahead, you may see a car slowing for a left turn early enough to change lanes. You will know about upcoming red traffic lights and be able to slow gradually, possibly avoiding stopping altogether. Watching ahead also improves safety as you will be able to detect dangerous situations as they develop.
When you are behind the wheel, try to consciously be aware of where your gaze is. If you are spending most of your time with your eyes fixed on your instrument panel, the steering wheel or the tail end of the vehicle in front then you have a bad habit that should be corrected. It can be tough to break this habit, especially if you picked it up when learning to drive. I have noticed that beginning drivers who concentrate too much on managing their speed tend to suffer from this. They spent their formative hours looking down at the speedometer needle and not up at the road.
If you do a self diagnostic and find yourself with this bad habit, it could be that you have bad eyesight. Get your eyes tested. People who have trouble seeing objects at a distance also often display this bad driving habit. Cases where a person's eyesight slowly gets worse unnoticed are especially pernicious. Failing eyesight can cause you to look at the instrument panel or the car in front gradually, because it is easier to look there.
Periodically scan the road out to as far as you can see it and then back. Your scan pattern has to include the instrument panel, the rear view mirror and the vehicle in front of you. But the main focus of your driving eye scan pattern should be the area of the road where you will be in 5 seconds.
So look ahead, be safe and save on gas!