This is an update of a post I wrote some years ago. Since I've been getting a lot of questions lately from writers wondering how to request contract termination and rights reversion, I thought it would be useful to take another look.
There's no "right" or "official" procedure for a rights reversion request. If you do a websearch on "rights reversion request" you can find various pieces of advice from authors and others. That said, here are some common-sense suggestions for how to go about this (Obligatory disclaimer: I'm not a lawyer, so what follows is not legal advice.)
First of all, if you have a competent agent, discuss the situation with your agent and ask them to approach the publisher on your behalf. Especially if you're with a larger publisher, your agent is more likely to know whom to contact, and in a better position to push for a response.