More troops... like 10-ish troops.
A city will revolt if:
Your culture is significantly lower than the other civs culture (IE. You have made less culture than he has made in that city)
The city tile is within the cultural influence of another city of that same civ (Imagine if every other city disappears. If the borders from the culture of a city would cover the city tile, then the city can revolt). As a result, even if you have influence over the entire BFC of the city, it can still revolt, if a neighbouring city has enough culture to reach to the city.
City revolt chance is reduced by the presence of military units. 10 units is enough to prevent most small revolts. 15 units, from memory, is enough to prevent all revolts.
It doesn't matter what the units are... 10 warriors work in the same effectiveness as 10 mechinised Infantry. Also, unusually, it doesn't even matter what civ owns the units... if there are 5 units that you own, and 5 units that (for example) Carthage owns, it counts the same as if you had 10 units on that city.
That is revolts in a nutshell.
Additional information, not critically important to know about, but related:
If the city was built by the same civilisation that owns it (IE. No civs have captured it), the city can flip (change ownership). You must have the cities flip from culture enabled (It is enabled by default). Also, if you select "Captured Cities flip from culture (disabled by default), then ANY city can flip culturally, regardless of whether it was aquired by force, or not.
Cities flip automatically after 2 revolts, if they are eligible to flip (Which, in this case, they are not)