This proposal is for Stack Overflow only, where questions usually get an objective answer, and where the answer rate is pretty good.

If a user has asked several questions, and hasn't accepted a single one, ever, after several days, then provide a message (that can be dismissed) at the top of the screen saying

> You have not accepted an answer to any of your questions. Consider accepting an answer if it has solved your problem.

Similar proposals:

 - http://meta.stackexchange.com/questions/80084/getting-new-users-to-accept-answers - different threshold, doesn't allow the user to dismiss the message
 - http://meta.stackexchange.com/questions/32553/dont-allow-people-with-very-low-accept-rates-to-ask-questions - different threshold, forces users to improve their accept rate rather than suggest it
 - http://meta.stackexchange.com/questions/69775/remind-new-users-to-choose-some-answers - different threshold, forces users to improve their accept rate rather than suggest it