It's usually best to cut taxes in a city rather than opting for a large garrison. The increased unit upkeep will outweigh the revenue gain and you'll end up making a loss. With taxes lower, your city will grow (despite what everyone tells you, this is a GOOD THING), and revenue will rise. You make more money out of a 30,000-pop city on low taxes than a 15,000-pop city on very high. City pop will eventually peak, then briefly fall, then stabilise.

For the same reasons, build farm upgrades.

Personally I never go above ten units of garrison except in very rare cases of huge cities very far from my capital. Since learning how to manage cities properly, I have never had a revolt except within the first 2-3 years after conquest.