I'd say the 3 best ones are: Exhaust (I use Magnaflow... saw a small increase), Intake (AEM Brute Force has been shown to be one of the best for airflow), and a tune (Blackbear is a site sponsor and have received MANY recommendations from site members).
Also, a few other things that are not necessarily mods are:
-Tire pressure (I saw 1.5MPG increase on my S10 by adding 15lbs in my tires to get the proper pressure... 20psi to 35psi)
-Plugs / Wires / Tuneup - If your vehicle has quite a few miles, you may see an increase in performance as well as fuel mileage with a tuneup. A good platinum plug (I used the NGK TR-5 in mine) will be all that's necessary. Other gimmick plugs (E3, Bosch+4, Splitfire, etc) have been known to work, but also have been known to cause more problems than they're worth.
- Fuel filter - this should be changed every year. Most people don't. Clogged fuel filter can cause performance and fuel mileage issues.
-Seafoam - Sold at most autoparts stores. Using Seafoam cleans carbon deposits in the engine. Do a few searches to find good writeups on this. It's pretty easy and cheap.
and as you said... driving habits. Light-footed driving is good, however that's not the entirety. Driving so that you don't have to stop at lights (slowing down early before a red light) will also increase mileage. I've found that if you can get to speed, then coast as much as possible, it will gain MPG. There are many driving techniques that can also add some fuel mileage as well.