According to an article I just read in Multi-Housing News online, potential renters put more faith in online reviews than the recommendations of friends and family.
Do you have a mechanism for online reviews of your properties? Even if it is just a simple YouTube video (free and easy) or a blog site for each property (free and easy) you should do this. Not only will it help with rentals, but the more reviews you have, the better search engine placement you receive when people look for rentals in your market area.
Yes, some unhappy tenants (probably the ones not paying their rent) are going to post negative reviews. That should be VERY rare. For legitimate complaints, the trick is to immediately respond with a solution to the problem, and grab an opportunity to be a hero! If the complaint is just plain crazy, or untrue, then comment that the person seems to be having a unique problem that is not shared by any other tenants. Offer to help find a replacement tenant so they can be let out of the lease early. Then, IMMEDIATELY, get all your happy current and former tenants to post comments about what a great landlord you are. Those comments will bury the older, negative, one, and push it down to page 2 or 3, where most people never look.
If you want to read the Multi-Housing News article, it is HERE.