1) The protestant and reformed movements are a great opportunity for the emperor to rack up ungodly Imperial authority. All you have to do is force your lambs to convert to your religion, doesn't matter which. (Well, it has to be christian.)
2) Always conquer the provinces from the "Imperial Ban" causus Belli. If you don't want all those provinces, you can often release a bunch of imperial princes for more authority.
3) Always answer the call to war.
4) Make use of "Imperial Liberation" causus Belli to liberate imperial princes.
The problem is usually getting the imperial princes to agree with the reforms, not getting Imperial Authority. Make sure your lambs love you to death, have royal marriages with you, and are the same religion as you before you waste authority on reforms that fail. Also, take your time. Try and make a beautiful borders HRE, don't rush it. After you form the HRE, you will have 50 years of chaos trying to keep your new country together, and then you have an unstoppable superpower that rules the world and can destroy anything with minimal effort and you win the game. I usually like to delay the last couple reforms simply because afterwards is far too easy and anticlimactic.