Some 88% of property managers raised their rent in the last 12 months and 68% predict that rental rates will continue to rise in the next year by an average of 8%, according to a survey of more than 500 of Rent.com's property management customers, which the site says represents thousands of rental properties and hundreds of thousands of rental units. That's nearly three times the wage increase that most employees can expect this year.

What's more, 55% of property managers said that they are less likely to offer concessions or lower rents in order to fill vacancies. One reason why they're getting even tougher: They are in a stronger position than they were this time last year.

More than 46% of property managers surveyed reported a decrease in rental vacancies in Rent.com's survey and, in the second quarter of 2015, vacancy rates in the U.S. for rental housing was 6.8%, the lowest it has been in almost 20 years, according to data from the U.S. Census Bureau.