Drive responsibly.
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.