Many have been said and I won't repeat them. But do save time by downloading an APP named "Metro" (yes, it's that generic) and allows you to swipe and go at subway stations instead of having to by tickets at machine booths each time.

Also, when you are transferring "within" a station, keep in mind that some of the transfers are literally two completely separate stations that they've simply built a tunnel to connect them. I.e., a transfer may not be as easily as you think (especially when you travel with young children or strollers as come tunnels have only stairs, no ramps).