Writer Beware often hears from authors who've signed up with bad or inexperienced or dishonest publishers, and are desperate to get free. They write to us wanting to know how they can break their contracts and regain their rights. Unfortunately, there's usually no easy answer to this question, even where the publisher has clearly breached its contractual obligations. Too often, I have to tell people that they are probably stuck.
That said, here are some general suggestions, which may or may not be applicable to your situation, and may or may not work for you (obligatory disclaimer: I'm not a lawyer, so what follows should not be construed as legal advice).
1. First and most obvious, check your contract for a termination clause. If there is one, invoke it per the instructions. Beware, though, of termination fees, which some publishers use as a way to make a quick buck off the back end.
2. If there's no termination clause, try approaching the publisher and simply asking to be released. A publisher may refuse or ignore such a request--but sometimes it will recognize that an unhappy author isn't an asset, and may be willing to let him or her go.
If you take this approach, don't dwell on the problems you've had with the publisher. Try to keep your explanations as neutral as possible--such as saying that you don't feel you have the time or resources to help promote your book, or pointing to falling sales. If you feel you must mention problems, do so in a factual, businesslike manner, without recriminations or accusations. Especially, don't mention any negative information you may have found online or heard from other authors. As large a part as this may play in your desire to be free, your request is about you and your book, not other authors and their books. Bringing others' complaints into the picture is likely to alienate or anger the publisher, in which case it may be much less disposed to pay attention to your request. (In some cases, it may become twice as determined to hold on to you.)
Another thing not to do: informing the publisher that it's in breach, and that you're terminating the contract yourself. This doesn't work for two reasons. First, even if you're correct and the publisher has breached its obligations--and even if the contract includes a provision for termination due to the publisher's breach, which not all contracts do--you, personally, have no way to enforce a termination. The publisher can simply deny your allegations, or stick its metaphorical fingers in its metaphorical ears and go right on producing and selling your book.
Second, you may consider the contract to be null and void, and your current publisher may not have the resources to sue you if it disagrees--but if you want to re-publish, you'll have problems. Another publisher won't be interested in a book whose rights aren't unambiguously free and clear. Even self-publishing services require you to warrant that you have the right to publish.You must be able to show some kind of formal rights reversion document--which you won't be able to do unless your publisher actually consents to let you go.
3. If you're a member of a writers' group, they may be able to help. For instance, SFWA has Griefcom, which will directly intercede in an attempt to resolve the situation for you. Similar services are provided by the National Writers Union's Grievance Assistance program. Novelists Inc. has a legal fund, which entitles members to up to two billable hours of legal consultation per year.
4. If there's no termination clause and the publisher refuses to consider a release request, you can resign yourself to waiting things out, either to the end of the contract term, if the contract is time-limited, or until the publisher declares your book out of print. Obviously this is more feasible for relatively brief terms of one to three years, and less so for longer terms, or for life-of-copyright contracts--especially since so much publishing now is digitally-based, and with digital publishing there's little incentive for publishers to take works out of print. Depending on your situation and your finances, however, it may still be preferable to the final option....
5. Consult legal counsel about your situation, and your options for taking legal action. This is where the issue of breach becomes relevant. A publisher may ignore an author's personal claims of breach, but may pay more attention if an attorney is involved.
If you choose this option, not just any lawyer will do. You want someone who practices publishing law. Publishing is a complicated business, with practices and conventions that are not well-understood by people in other fields; and publishing contracts are unique documents with terms and conditions that aren't found elsewhere. In order to provide effective representation, your lawyer needs the appropriate skill- and knowledge-set.
(This same caution, by the way, applies to hiring a lawyer to vet a publishing contract prior to signing it. I hear from any number of writers whose non-publishing-specialist lawyers gave the green light to a contract that would never have passed muster with a publishing law specialist, or a competent literary agent.)