As the alliance with Poland sooner or later breaks, only use them in the first war against the Orders (take Neumark and Danzig) and the vassalization of Pommerania (sell them Danzig).
At this point Poland usually leaves the alliance, which is just what you want (as long as you still have Bohemia and Austria). Wait until you spot a weakness (if that takes too long wage another war against the TO) and hit them hard by using your allied forces to preserve manpower. If Mazovia is still alive you can take them as a vassal, otherwise you have to take a province, release them as a vassal and then return their cores in the next war. Don't take too many provinces from them, as the vassalization/conquest of the Orders generates some AE as well.
If you can conquer the Orders (usually Denmark attacks the Livonians and reduce them to force-vassal-size) and reduce Poland in size while maintaining the alliances with Bohemia and Austria, the east is yours to take. Consolidate a little at this point, build up your trade fleet and army and prepare for several beatdowns of Poland/Lithuania.
After the vassalization of Pommerania you should also be able to start diplo-annexing small nations within the HRE (Anhalt is the first possible), which is a great way to expand there. Also pay attention to the blobbing inside the HRE: If some nation reaches 4 provinces, attack and vassalize.
Edit: I'd advice to vassalize both the Livonian and Teutonic Order as soon as warscore allows it. Preserves a lot of adm points and uses much less AE than an entire conquest.