A few things I haven't seen others say:

Only join your allies wars as a last resort. Don't dishonor the call (unless honoring will wreck you), just avoid it. This starts by keeping your allies busy in wars so they won't declare as many of their own. In the mid game if you have 2 great power allies they'll often end up in wars inadvertently if you let them (the AI is much less likely to declare wars if they have loans/WE keep them busy fighting your battles). The next step is to declare an independent war yourself against the target, if you are going to fight, you want to set the terms, the AI will only screw you in the peace deal. The next option is to dodge the war, declare a minor war where you are fighting vs one of the co-belligerents on either side and you can "accept" the call with nothing happening. This has saved so many headaches. Ex: Sweden (my ally) declares war on denmark, but wait, france (my other ally) is defender of the reformed faith. Luckily, denmark is also allied to the Livonian order. I declare war on the Livonian order, and only have to fight them, denmark and a weak pommerania. My alliances with sweden and france are both intact, and I am fighting a relatively easy defensive war.

A corollary, to the previous tip, keep an eye on your allies troops, they will usually move them in position before declaring war, you can often jump the gun on them and declare so you are in charge of the peace deal.

Break coalitions as they form/prevent them from getting large. A nation can't join a coalition if they have a truce with you. You can also currently get 100% vs a coalition by completely occupying the war leader. Yes, some people find it easier/better to keep AE down, but some games its just unavoidable (like when you are forcibly eating the HRE). As soon as a few countries join the coalition, declare on them to break it up. Did you let it get too big, maybe declare on one of the stronger members allies and pull them out of the coalition in the subsequent truce. Things really getting hairy? Go YOLO on the weakest/most isolated coalition member (i try to get at least one nation pissed off at me on the front im not expanding on via insults, fabricating claims and being discovered), get 100% on them as fast as you can and peace out.

Keep a whipping boy OPM around, sometimes you will need to be at war (being at war prevents you from falling into a PU and prevents the emperor from demanding unlawful territory, i've declared on a OPM astrakhan that i completely surround probably 12-15 times in my current game with the holy war casus belli just to avoid unlawful territory demand, once i core all the territory i take i peace them out for nothing).