You would need at least pre-Napoleon POD avoiding unification and even that would be probably bit hard. Best what you can do is delay unification with some decades.
I would say, that it is far easier before Napoleon, but a POD after Napoleon is still possible, while the probablity shrinks with every year.
Prussia could loose several wars, while Austria could remain weak by loosing their own wars.
After the Austro-Prussian war, any balkanization is only possible through foreign intervention and enforced balkanization.
The easiest way would be, that Prussia continues to controll Warsaw (like a short time before the Napoleonic Wars) while Austria transforms into some Austro-Hungarian-Croatian-Bohemian Federation. you also need to prevent the formation of any larger state in Western Germany.
Another way would be to form several medium-large sized states which balance each other. (Basically a larger Saxony, Bavaria, a little bit smaller Prussia, Hannover, An independent state in Westfalia and the Rhineland, and Baden and Württemberg in the South). If alle these medium states have a similar size, any unification would be far more difficult, since there would be no hegemon.