Controlling your CityVille population is just as important as controlling the population in a real city. As more and more people move into a city, it becomes necessary to increase community services and provide businesses where those people can buy whatever it is they need. But if you increase those services and businesses faster than you increase the population, then there won't be enough people to support them.
When playing CityVille you have no choice but to balance your population growth against the growth of businesses and community buildings - the game forces you to control each of these numbers before you can expand your city. And nothing is more frustrating than to win that cool CityVille Beach Hotel and not be allowed to place it because your population isn't large enough for expansion.
Each new expansion requires a certain population level and it usually means a really tight squeeze on the mean streets of CityVille. Eventually, though, you're going to hit the higher levels of game play where it's just not possible to squeeze in even a little Red Dog House and there's no way you can ever meet that population requirement. How then, do you increase your CityVille population, when you have no more room to build housing?
Collect rents: The easiest solution would be to just be patient and continue to collect rents on all the housing you already have. It's a random occurrence but CityVille does occasionally reward you with a population increase if you're regularly collecting your rents. However, it doesn't happen every time and it might be months before you get enough population boosts to enable you to expand.
Upgrade housing: If you've reached the point where you don't even have one, single space left on your grid and you still can't expand because you need more population, then it might be time to start replacing some of your smaller houses. Remove/Sell some of your smaller houses that only allow a 10 or 20 population and start replacing them with high-rises, mansions, or other housing opportunities that will allow for more population.
These smaller homes might look really good when you have a whole block of them but when you consider they Energy you spend trying to keep the rents collected, they're really not that beneficial once you reach a certain level of play. Look at it like a real estate developer would - every day you see older, smaller homes being torn down and replaced with apartment buildings and condos that will house a larger number of people and produce more income.
Upgrade your CityVille Mall: Upgrading your CityVille Mall is a major undertaking. Each different level requires a certain number of items - items you have to beg for from all your neighbors. But you can maximize your much-needed space and increase your CityVille population when you upgrade your mall to its fullest potential. At Level 5 your mall will hold 20 businesses which will save a ton of space in your City. And you lose nothing by moving your businesses into the Mall. In fact, as you upgrade, you actually increase your earnings because the mall pays a bonus percentage each time you collect.