Roads & rails
1. Creating a highway that will be the "spine" of your city can be beneficial for the traffic flow; however, a proper road hierarchy is needed in order to maintain the flow in the city and on the highway.
2. Sometimes you will need a way to diffuse heavy traffic from the highway/freeway; for that you can use frontage roads or local-express roads.
3. The fewer intersections the better. Also try to avoid traffic lights as they disturb the flow. You can do this by, for example, transforming the road to highway.
4. Strategic roundabouts shouldn't be small (the smaller the roundabout, the more the cars have to slow down, thus causing a decrease in the flow of traffic).
5. Use one-way streets whenever possible, especially in industry districts, as there is a tendency for trucks to clog the roads.
6. Remember to connect two lines of highways in a way that people can turn around easily without needing to enter the city.
7. Too many roads merging in a small area is bad for traffic flow.
8. Try to split your truck traffic away from your commuter traffic. Having both loads of traffic on the same roads can quickly snarl even well-designed traffic systems.
9. Use cargo rail in industrial districts—they will greatly help develop the district and remove part of the traffic. Remember that a good road system is needed to serve the cargo station, as every time a train arrives, a lot of trucks are moving the goods to their destination. (One-way streets are one of the solutions here)
10.Great ideas for intersections can be found here: www.autobahnkreuze-online.de Keep an eye on the Steam Workshop as some of them will most likely be posted there by our awesome modders!

Public transportation
Start the public transportation as soon as possible, and try to keep the lines simple.
Metro is the most effective transportation system, but it is also quite costly. A good mix of buses that complement a metro system is the most effective overall solution.
Passenger trains are currently not especially useful for transport within a city, though can be useful for bringing tourists to the city.

Services
1.Note that garbage and dead body disposal does not have infinite range, so you will need to disperse garbage services and body disposal services throughout the city. The associated infoviews will show you which parts of the city are within range of disposal services.
2.Don't put water intakes downriver of sewage outlets.
3.Parks and services increase land value, increasing popularity of given land, as well as stimulating those areas to develop.
4.The most optimal pipe coverage is $440 worth of pipe between two parallel pipes, which will result in the smallest possible amount of overlap between the zones of coverage of the parallel pipes, whilst still ensuring everything gets served with water and sewage.

Planning
1. Make use of districts and city policies. More here: Dev Diary 8: Districts & policies
2. Try to separate pollution-heavy buildings/industries from residential areas.
3. Agriculture and forestry industry produce no pollution, and you can utilize them even if the area doesn't have natural resources of that type. (They'll just import what they need.) You can use these as buffer zones in between your industry and commercial zones. They do have significant noise pollution (as does all industry), but commercial is much more willing to put up with noise pollution than residential is.
4. Pedestrian paths: Just because they are under the "decorations" section does not mean you should write them off as just decorative fluff. They are legitimately useful in cutting down your traffic. A resident walking to work is one less car clogging your road. It's like free mass transit. You can also make pedestrian paths elevated the way you can elevate roads.
5. Building orientation tip: The orientation between two roads that are in a 90-degree angle to each other happens by moving the mouse pointer towards the road that you want the building to face. So if you have a corner with a large road and small road and you want the shopping mall to face the large road, you can first move the mouse to the spot you want it in and then slightly move the mouse towards the larger road and the building will orient towards that.
6. Trees can reduce noise pollution.
7. Offices can be used as a barrier between commercial and residential areas to reduce noise in residential areas.

Other
1. Save your city before you build a dam as sometimes the results might be different from your expectations, and dams are one of the most expensive buildings in the game.
2. Browse the Steam Workshop for cool mods, as well as fancy intersections that will, in general, improve the flow in some critical places.
3. A great guide from SimNation - simnation.tv/citiesskylines/guides/cities-skylines-guide-for-existing-city-builder-players/
4. You can use terrain.party/ to obtain a height map of you city of choice and import it into C:S. Instructions are posted. here.
5. Don't buy something until you need it—most things have an upkeep cost. If you can get by without it for a while, you will build more reserve money when you need it. The exception to this is the beginning of the game where you want to expand quickly to put your budget in the green.