I'm a bit puzzled bythe long fermentation times some are mentioning. To me Berliner Weisse should be clear, crisp, dry, sour and refreshing. While some historic Berliners had a combination of bugs, including Brett from barrels, modern ones (Kindl Weisse) are only soured with pediococcus, which works fairly quickly following primary. The beer is wonderful consumed young! this is not lambic with a German accent!
For a bit more complexity, I usually sour with a mixture of live yoghurt (just dump a cupful in your secondary) and/or a couple "pro-biotic" food supplement talets dissolved in a little warm water. In both cases the prominent bugs are Lactobacillus spp. rather than pedio, but they do make a nice clean sour.
Aother approach I have success with with all-grain Berliners is to pull a quart or two of the mash, let it sour mash for a day or two or three, then heat it to about 165-170 and hold it there for 15 minutes to pasteurize. I then sparge this with a bit of warm water and add it to the fermenting brew. It's relatively easy to control the amount of sourness using this technique.