Fast-growing cities face similar problems to those of fast-growing startups. Both struggle to maintain their original character and to not let their resources stretch too thin, especially during growth spurts. 

The Wall Street Journal recently sought to identify the world's most innovative cites, asking a dozen architects and urban planners for their input. Like startups, these cities are dealing with what the WSJ calls "the eternal struggle to balance growth and quality of life."

While it's mostly the world's largest cities that dominate the conversation around urban innovation, the cities that the experts identified as "worth watching" are mostly mid-sized. These cities that have done a good job of identifying one problem that plagues them and honing in on the best ways to solve it. Here are three lessons that you can learn from five cities that are "leading the way in urban innovation."

1. Identify your biggest restraints.

Singapore earned a spot on the WSJ list of cities "worth watching" for finding creative ways to manage its severely limited resources. Located on an island approximately the same size as New York City, Singapore had to find ways to limit congestion as its population swelled, and also to cut down on the amount of water it imports from nearby Malaysia.

In order to ease congestion, the city implemented a "congestion pricing" system that requires motorists who enter the city during rush hour to pay a fee. Singapore is also exploring a variety of ways to extract more water, including collecting it from the runways of its main airport, Changi, for irrigation use and building two desalination plants on the island that can produce about 100 million gallons of clean drinking water each day.

2. Affordability is critical to maintaining growth.

One of the biggest problems that has plagued fast-growing cities such as San Francisco, London, and New York is shrinking access to affordable housing. As a result, population growth is slowing, and some of the cities' longtime residents are fleeing.

Houston is trying to combat this trend by implementing a number of pro-growth policies. It's one of the only major cities in the U.S. to have almost no formal zoning laws. This means that developers can build a townhouse or an apartment complex in whatever area of the city they please, and they don't have to go through expensive rezoning processes.

It looks like the relaxed policy has paid off. According to the WSJ, Houston had a 6.7 percent increase in population from 2010 to 2014, second only to New York. 

3. Direct some of your resources into revitalizing neglected areas.

Two of the cities that made the WSJ list of those "worth watching" include Medellín, Colombia, and Detroit, for their efforts in developing neglected areas. Medellín added a system of outdoor escalators in some of the city's worst neighborhoods, in order to promote tourism in these areas.

Detroit, meanwhile, is taking a cue from Houston and trying to relax its zoning laws. The city wants to ease bureaucratic constraints in some areas, called "pink zones."

Maurice Cox, a planning director for Detroit, told the WSJ that the city's goal is to create a place for entrepreneurs where "you won't have to go through months of red tape" to build a business.