Accelerate modestly - even if other drivers behind you don't like it. Be disciplined and drive in the 'slow' lane. (remember when there was a lane for passing and a slow lane? That's ancient history, I guess). Coast when possible. - use brakes only when really needed - don't rush up to a stop. Keep tires properly inflated and buy tires with a Green Energy certification. Drive at the speed-limit. Use Ceratec in the engine oil to reduce friction. Use it in the differential also, but not in the transmission unless it is a manual.
Don't idle the engine. (Some manufacturers now kill the idle at stoplights). Jettison excessive weight - stuff you don't need to carry around. Don't install roof-rails to cause drag. Wax the vehicle frequently to keep the surface as slick as possible. Exercise yourself - lose weight - same for the wife or husband or kids. In a few years, over half of amerikans are on schedule to become obese.
It takes more fuel to move fat, right?
Rowdy Lee summed it up pretty good. The only thing I do from his list of items is drive responsibly. I don't worry about mechanical modifications, aside from using good quality "Amsoil" oil and good quality gasoline. In fact, I've added probably about 30-50 pounds of sound deadening material to my HR-V in order to get a quieter ride (OMG, the HR-V is noisy as an avalanche in a heavy downpour without sound deadening butyl sheets added, lol).
If you see the light is red up ahead, get off the gas ahead of time and coast, aim for trying to prevent yourself from coming to a complete stop before the light turns green if possible. Accelerate slowly, stay in the granny lane and do the speed limit. I did an essay for college on fuel efficiency, and for every mile you go over 55 was like adding another 40cents to every gallon of gas (the essay was around 1998, now it would probably be like adding another dollar for every gallon) simply due to the increased air resistance which increases exponentially over 55mph.
I live in a very mountainous area of Easter Pennsylvania, and with sensible and reasonable driving habits I've been able to average about 36-38mpg in the summer and 32-33mpg in the frigid winter using regular unleaded gas with 50/50 city/highway travels in my 2018 HR-V EX AWD.