Troubleshooting anti-forgery token problems

I have a form post that consistently gives me an anti-forgery token error.

Here is my form:

@using (Html.BeginForm())
{
    @Html.AntiForgeryToken()
    @Html.EditorFor(m => m.Email)
    @Html.EditorFor(m => m.Birthday)
    <p>
        <input type="submit" id="Go" value="Go" />
    </p>
}

Here is my action method:

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Join(JoinViewModel model)
{
    //a bunch of stuff here but it doesn't matter because it's not making it here
}

Here is the machineKey in web.config:

<system.web>
  <machineKey validationKey="mykey" decryptionKey="myotherkey" validation="SHA1" decryption="AES" />
</system.web>

And here is the error I get:

A required anti-forgery token was not supplied or was invalid.

I've read that changing users on the HttpContext will invalidate the token, but this isn't happening here. The HttpGet on my Join action just returns the view:

[HttpGet]
public ActionResult Join()
{
    return this.View();
}

So I'm not sure what's going on. I've searched around, and everything seems to suggest that it's either the machineKey changing (app cycles) or the user/session changing.

What else could be going on? How can I troubleshoot this?