1. Go underground
Busking in subway stations and connection tunnels where it's warm and full of people waiting for trains is a great solution because you have a captive audience and your not freezing. Usually you're allowed to play in these public spaces as long as your music is at a reasonable level. Your music can't be louder than the announcements at the station where you are playing. But don't be too quiet, you want to find the perfect volume to be noticeable but not disturbing.
2. Rules and regulations
Before you go underground it's a good idea to find out what the rules are for your city. Usually you will need official permission to play and you won't be allowed to play with drums, brass, or amplification in train stations. You are not allowed to play in the trains themselves, though people do it all the time, it can be annoying for the passengers. Not following the rules could get you a huge fine and perhaps even get you blacklisted as a busker.
3. First come, first serve
In Berlin as of 2017 you can get permission to busk on Wednesday's at Leopoldplatz U6 station between 7 and 11 in the morning. There is a line by 6.30 in the morning and it is first come first serve. It is a bit like going to the doctor or the immigration office, you get a ticket with a waiting number and wait to be called. They don't check the quality of the music like they do in other cities with different laws but you must pay 7.50 Euros per day for the following week and only play in the stations where busking is officially permitted between 6am and 10pm. This 7.50 per day ticket to play also includes arrival and departure trips on the train, which is great especially in the winter.
4. Find your hot spots on the map
The hot spots to play are always in the tourist districts of any city.
Here is a map of stations you can play in Berlin. To get these spots you have to get the sign up early. Usually there is such a thing as busker etiquette where performers won't hog a good spot the entire day and the musicians share the bounty of a busy location. But then again there is also the occasional busking mafia who may try and bully you out of a spot. Stay fair to your fellow buskers and report bullying.
5. Follow the holiday spirit
A good way to fill your coffers is to plan to busk around the holidays and around holiday markets in general. Bring your merch, know how to please a passing crowd and learn some festive tunes that people might know and may automatically hum along with. The winter holidays are one of the best times to make money as a busker because some people are in the holiday spirit with a generous mood and might drop you some extra cash and may buy your albums as gifts.
6. Safe and sensible
Keep your cash in a safe place and be alert for thieves. Don't use expensive instruments that might break from the changes in temperature and humidity. Unless you have insurance get yourself a cheap instrument that you can use specifically for busking. If you don't want to get sick from the exposure to the weather and crowds, be sure to dress properly, stay hydrated, take Vitamin C and use disinfection sanitary gel after handling money that people give you. Above all, have fun performing!