You can sell units for a function of half their base cost in gold at any time. Any worker or settler that will be captured next turn can be sold instead. Any worker or settler that can be captured but not protected, can be sold immediately. Before costs rise, in the early game, it's actually worth it to sell workers after using all but one of his build actions. Then you can usually immediately buy a new worker. You end up paying 1/6th of a worker's cost to get 4/6th extra actions, a good deal in my book.
Cards that improve wonder building by 15% are not worth it. You're better off with +30% worker building, as workers can usually chop trees to speed up that wonder building far quicker than the 15% boost would.
Having a couple of farms next to each other really pays off in the long run. Every 2 farms give an extra housing. If any city doesn't have 2 unoccupied housing, its growth is severely reduced.
Besides the housing it helps grow the city to work more tiles, provide more science and culture. Getting feudalism boosts this food production from farms. Getting replacable parts boosts this food once again.
For the city planners: District adjacency also works for districts from other cities. I was playing as germany and their specific industrial zone (called hansa) gets +2 production if next to a commercial zone. This led to a nice diamond of 2 hansa's and 2 commercial zones, giving each hansa +4 production. This is particularly useful as late game there are industrial zone buildings that give a bonus to nearby cities. I am planning in the future to have 3 cities who combine resources this way. They'll have 1 hansa next to three commercial zones and 2 hansa's next to two commercial zones.
It's good to maximize value of trade routes, but don't forget to use an occasional trade route to get a road where you need a road.
The harbor adjacency, +1 gold for each coastal resource and +1 gold for every two adjacent districts may not look like much, but remember that one of its future buildings gives production equal to the harbor gold bonus. Ideal is to plan a city center/harbor/commercial triangle, as this benefits both districts quite comfortably.
Although it is busy work, in the long run trade routes can give tons of bonuses. It doesn't matter how long the route is, it doesn't matter where the route is from, the only things deciding the gains are the city that they're going toward, your policy cards and your wonder/civ bonuses.
It can be a good idea to tailor your strategy based on nearby city states. I once had three building oriented city states. I focused on building multiple cities with an industrial district and trying to complete as many of their quests to get 6 envoys to each. At that point I got +12 production at each industrial zone city, as long as I was building districts, wonders and buildings. I ran out of things to build real quick so I sent out settlers to some of the least habitable places and then used traders towards my advanced cities to quickly build these up. I imagine similar things can be done if you meet 3 religious, 3 military or 3 commercial city states.
The city state bonuses surpass the bonuses that most civilizations give, so that I think your playstyle is decided more by the nearby city states than the civ you're playing.
Unless you're playing as greece, political philosophy should be the first goal of the civics. Greece with some justification can go for mysticism or military tradition, because this allows them to crank out a prophet or general before anyone else with their free wildcard policy. The other civs have no use for this until they upgrade their government system.
