system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] How does Sauron regain strength after his defeat?

Is it just a simple timer for his spirit to recharge until he can retake physical form? Does he do anything to get stronger faster?
gpt
There is one thing you should bear in mind: if you're most familiar with the movies, then you're probably working off the assumption that, after he lost his Ring, Sauron lost his body and became a giant flaming eye ghost. 

That's not suggested in the books. They mention a flaming eye, yes, but in context it seems they're just talking about a heraldic sigil or something. A metonym. 

By all indications, Sauron didn't lose his physical form, but, demoralized and having lost his Ring of Power, retreated to lick his wounds for a bit from his catastrophic defeat. 

The "power" he's growing in is probably political in nature; alliances formed, siege engines manufactured, ranks of orc swelling and brand new deadly breeds hitting the market.