1) Drafting: close behind
At highway speeds there's no doubt that driving close behind a large vehicle dramatically reduces fuel consumption.
This is not a recommended practice due to it being illegal in most areas.
That being said you CAN draft without tailgating. If your local law says you must be 3 seconds behind the vehicle in front of you then stay 3 seconds behind the semi. You may not be getting the same benefit as true drafting, but it's better than nothing.
2) Find a 'blocker' for slower freeway speeds
Some people are uncomfortable driving at speeds less than the average flow of traffic on multi-lane freeways.
One solution is to find another vehicle going the speed you want to travel (large, conspicuous vehicles work particularly well) and drive either ahead of or behind it.
3) Close the sunroof at higher speeds
Some sunroof styles are better than others. The worst offenders are the kind which tilt and slide to the outside, on top of the roof. When open, these "roof-top spoilers" can significantly increase aerodynamic drag.
4) Reduce speed
Aerodynamic drag increases exponentially with speed, so reduce highway cruising speed as much as practical and safe.
Generally, a vehicle's most efficient speed is just after its highest gear has engaged.
5) Constant throttle position cruising
Once up to speed, pick a throttle position and hold it.
Advantages: more efficient than using the cruise control (which varies throttle position frequently and wastes fuel on hills).
Disadvantages: less efficient than "driving with load" (DWL) / "target driving" (where the throttle is eased on inclines).
6) Cruise control - when to use it
Set the cruise control if you're the type of driver whose speed creeps up higher and higher the longer you're on the road, or if you have difficulty holding a steady speed (it wanders up and down).
But realize that cruise control is just a band aid for those behaviors. Generally it's less efficient than constant throttle driving, and much less efficient than "driving with load" / "target driving".
7) Cruise control - when not to use it
Only use cruise control on flat roads. On hilly roads, cruise responds to changes in grade - by feeding in more throttle on the uphill and releasing on the descent - in the exact opposite way an efficient driver would.
The 'corridor effect'
All else being equal, traveling at a constant speed on a freeway within a flow of traffic (in the same direction) is more efficient than going the same speed in isolation. The reason is aerodynamic: a flow of traffic generates a localized wind current in the direction of travel. You will benefit from this artificial breeze.
9) Drive by the MPG DIC
Start out by getting your Cobalt to the cruzing speed you want. Then begin to let off the throttle while keeping track of your DIC set to INST MPG. You are looking for the max mpg you can get while staying at that speed on LEVEL
Once you find this "magic" mpg number stick with it. This means change your throttle position as many times as you need to in order to keep this mpg number the same or more. You will find after a while that you are pressing the throttle more when going downhill. This is where you pick up speed.
You will also find that in order to keep that mpg number the same you will be letting off the gas going uphill. Don't worry, you will get that speed back when you go down the other side.
NOTE: Before you go driving by the MPG DIC set a hard limit. This means that when you slow down below a speed you are comfortable with you give it more throttle to get back to your cruzing range. You will find that you really do not need to do this often. After all, no one wants to be rolling up an interstate at 45 mph.