How can author deal with late reviews?
I have found that a polite, terse e-mail to the associate editor with a clear history line often works in getting some response.  Occasionally, I have written the editor-in-chief if an associate editor is ignoring me.  As Peter points out, there may be legitimate reasons for a delay, including:


The editor is having a hard time finding a competent reviewer (maybe the area is too narrow).
Your paper is very hard to understand (sometimes, if it's badly written but seems to have some elements of novelty, a reviewer will try to plow through to understand what is going one).
The editor needs an additional reviewer in order to break a strong difference of opinion between the original reviewers.


When even the editor-in-chief does not answer in a timely manner (and I have had this only once in my career), your best recourse is to never submit future articles to that journal.  You will be doing your community (and the next submitter) a service.