Ways to keep milk supply up after going back to work?

So I have been back at work for about a month and I have noticed my milk has slowly declined. I will either get just enough milk or will have to go into my freezer stash to get enough (I’ve been short 1-2 ounces more frequently now). I pump 3 times during the day and once in the middle of the night. My baby is being supplemented with about 5 ounces of formula throughout the day - I was pretty sure I would have to use more so I’m not going to change that. She does well with the formula and seems to sleep through the night since it fills her up. What are some secrets that have helped you maintain???