- stop using powers on something else;
- start using expensive advisers, promote them;
- stop paying with institution penalties;
- stop having bad rulers;
- stop getting bad events;
Last two you can't address much, that's where the game shows, how much it wants you to do nothing but watch time go. Another 3 can be addressed, depending on how badly last two go. To fulfill the second, get more money, it means more land or subjects that pay. The third can be addressed by development in your capital (develop it when new institution is happening), or find another source if you can. First one is super obvious. If you can't stockpile power, you're handling your land wrong.
If you lose your points from events, than you have to google, why. Game may be cryptic about reasons behind events, but not always. Once I lost about 2 techs worth of military power in 50 years due to "division of council" event playing as republic. Flavour text says that's it's because we go against our republic traditions. From wiki I learned that you can get that event if your republic tradition is below 80, which means that you can have a decent ruler only for 5 years every 15 years, and everything else is decided by a roll of dice (i.e. how often bad events gonna trigger). I tried to keep tradition in around 70 (in monarchy, bad number of legitimacy is bellow 60). This session I learned, how amazingly awful republic government can be.