City profitability (excluding construction costs, if your city takes in more denarii than it spends, the Prosperity rating will rise, and if it takes in less than it spends, the rating will fall) ;

If the city needs additional funds from Rome, its Prosperity rating falls;

Should the city ever fail to pay tribute to Rome, its Prosperity falls;

Wage rate (paying more than Rome boosts Prosperity, while paying less reduces it);

Housing levels (having quite a few villas in your city boosts Prosperity, while a large proportion of your citizens living in tents and shacks reduces it);

If lots of people eat more than one type of food, Prosperity rises;

The presence of an active hippodrome in a city boosts Prosperity.

The easiest way to boost Prosperity is simply for your treasury to take in more money than it spends, not counting construction costs (which are, after all, enhancing the city).