I have the same car, and the same commute.
Ideally, you'd charge it at home each night, and at work each day, and you'll barely use any gas. But like you, I don't get to charge it that much. Maybe 3-4 times a week at most.
So my best advice is, the more you can plug it in, the better. Also, keep your freeway speeds under 75mph. It's also worth considering putting the car in HEV mode on the highway, even if you have EV battery charge, because your EV charge is better used on city streets than on the highway, I've heard.
I also use the cruise control a ton. Nearly always when I'm on the highway. Some people say that hurts the MPG, but for me it's helped keep my MPGs higher.
With a similar commute schedule as you, I tend to get about 65mpg on average, with about 4 plug-in charges.