Ubahns. This is great if you are an acoustic player, but you can get away with using your amp aswell as long as it's low. You don't have to have a permit to play in some places and the BVG hardly ever go to Blissestrasse, Budesplatz, Gleisdreik, Westphalweg, Platz de Luftbrucke, Tier Park. But if they do, just greet them with smiles and the best German you can and alles will be klaar. See the previous guide to get a permit.
There is a big pedestrian area in Wilmersdorfer Strasse and is great for busking. Also there is a big walk way with lots of tourists just outside of Zoologische Garten. If you fancy a little trip just outside of Berlin then take the S1 to Potsdam and it is a lovely place to play. I wouldn't bother with Warschauer Strasse anymore, it is such a dark atmosphere there now. Schleisches the same. There are lots of nice spots in and around Prenzlauer Berg and usually just outside Eberswalder Str can be a good place, especially on Sunday.
If you enjoy playing in the trains then the S1 line seems to be the most popular. Get on, play a few songs and jump to the next carriage. I never felt the desire to busk this way, but lots of people do and make a good hat from it. A guy who does it told me he averages 40 an hour with his friend. I prefer to find an Ubahn and play for a few hours myself. Nice sounds in the Ubahns.
Hackesher Market in and around is much better than the cold Alexanderplatz. Go out of the market towards the lake and there is a great busking spot. Also on the bridge is a great spot, but usually there are 2 or 3 accordions within spitting rage holding that bridge for hours. But lots of places around here are good as its mostly commercial area and museums so there are no neighbours to annoy.