After struggling with this problem for a couple of years, and ending up creating a product with relatively poor usability in it, I've solved this problem a different way.
There are two sources (at least where I work) to the problem:
1) "The Business" will never schedule/prioritise usability issues as a) they'd rather have new functionality/bugfixes and b) they have no real way to conceive of or quantify the cost of poor usability (they're domain experts after all, not software experts).
2) The (traditional) developers will never schedule/prioritise usability issues as they a) often can't see the extent/cost of the problem and b) are often less confident in front-end development and c) their focus is constantly stolen by issues that are more core to a developer's job.
Consequently, usability suffers.
As such, IMO, you essentially need a separate channel of development to address usability. It's much the same as technical debt (which I would also advocate gets 'solved' this way). Create a new channel of dev, whether it's as small as dedicating a couple of agile points per iteration (or a couple of days per two week release, whatever) to, in an ideal world, hiring a front-end engineer with his own priority-list. This new channel of dev constantly and consistently churns through usability issues, and usability issues are no longer prioritized against non-usability issues but rather against each other.
Now, for both of the audiences in my first paragraph, if you go off and spend a couple of days collecting some research summaries, case study outcomes, quotes from the recognized gurus etc, you should be able to build an argument that will convince people. But who has the time to put in 4 hours of research and argument-building to sufficiently prioritize a 1 hour job? But if you get an argument together (citing a few examples of poor usability in your app) not in support of justifying the prioritization of a single issue but rather justifying a separate channel of development, the payoff is suddenly much more worth the effort.
I am an ordinary C#.Net developer, with an increasing interest in usability. What I did is formulate an argument for the importance of looking at usability issues, went to my CTO with a proposal, and now I'm dedicating a percentage of my time for usability issues. My team has lost some core development resources (we're were hiring so it was an easier argument to make anyway) but now some usability will finally get looked at.
Long-term I am thinking part of my new job is to educate the other developers in such a way as to not release features that exhibit poor usability. Amplify the effects, as it were.