Yeah by mid game you should know what victory you should go for and start focusing your citizens pretty exclusively on that. It can be good to have a backup victory type too just in case.
City placement is also something that's fairly important. If you settle in a place with tons of hills you'll produce like crazy but you'll have more limited growth due to lack of farms. Or vice versa if you settle in flat grasslands with not a hill in sight your city will grow nice an big but you won't produce jack shit.
You also need to consider luxury resources, strategic resources, rivers and coast when settling.
Also you only get a happiness boost from the first copy of a luxury resource you connect to your network. Let's say you start out next to two dyes. You improve the first one...+4 happiness. Then you go to improve the second one...nada because dyes are already connected. But if you spawn next to a dye and an incense then you would get +8 happiness for both. So when it comes to luxuries you're looking for diversity if you're having happiness problems. And don't forget to trade your "extra copies" of luxuries to other civs. They are no use to you just sitting there.