What I do is after I'm done building improvements in a city (toward end game I now have Aqueduct, Marketplace, Hospital, Courthouse, Mass Transit, Police Station, and Bank in most of my cities), and I switch it to wealth, I knock out all the mines and forests and convert to all irrigation. Those specialists might not add up to much in one city, but get say 100 cities with specialists, and watch your income or science fly.
Right now I've got a game where I'm getting almost 800 income alone just from taxmen and wealth.
More irrigation before this point occurs also will speed your growth up to that magic 20 number, and more population means more everything, in my opinion.
My civ's population last I checked was like over 350 million!