The only 2 things that have noticeably worked for me (in my limited experience) is to get slick tires and to reduce the weight.

I bought some lightweight tires that are still wide enough to soak up the bumpy roads. I was hesitant to get wider tires (700x35), but I'm glad that I did because they were noticeably faster than the semi-knobby tires I had which were skinnier.

If you already have slick tires, look at reducing the weight of anything and everything. I was surprised how much a pound or two made. As mentioned, I had lighter tires, but I also went through my bags and got rid of the little stuff that I really didn't need (but kept "just in case"). For instance, I had a flashlight that I can mount to my bike, so I did not need my little LED keychain light and the charging cable that I was also carrying around. After taking out a handful of things like that, I was surprised that so little actually amounted to a better ride...I just hope I don't become a junky obsessed with weight savings down the road. At some point, that ends up costing more than it's worth. $$$