My five year old, 5-speed normally gets 25-30 mpg highway, but I recently started using some "Hypermiler" techniques and found big improvements in my gas mileage (7-10 miles per gallon).  Here's what I am doing:

1. Pulsing- Once I reach my desired speed, I apply small bursts of gas to maintain a speed
2. Coasting- I put the car in neutral on downhills as opposed to driving (using gas) down hills (I only recommend this for people with manual transmissions)
3. Checking air pressure- Once a week I check my tire's air pressure and make sure it matches the auto manufacturers recommendations
4. 65 or less- I try to stay at 65 or less on the highway
5. Lightening load- I have removed excess weight (no junk in the trunk)
6. Slower starts- I am trying to ease up to speed as opposed to quick starts
7. No parked idling- If I will be sitting for more than one minute, I turn the car off
8. Selective air conditioning- I use air conditioning above 45 mph, under 45 I crack the windows
9. Higher gears- Once I reach my desired speed, I move to a higher gear to keep my RPMs lower thereby lessen the work the engine needs to do

Some "Hypermiling" techniques I won't use because I think they are dangerous or unreasonable:

• Ridge riding- Riding off center in the lane to avoid the ruts
• Engine off riding- Completely turning the engine off in a coast
• Over inflation- Inflating the tires to their maximums as opposed to the manufacturer's recommendation for the car
• Drafting- Following close to a vehicle in front of you to reduce wind resistance
• Too slow- Driving well below the posted speed limit

Use what works for you.