backbone-custom-method.js

var MyModel = Backbone.Model.extend({
    someMethod: function(opts) {
        var url = this.url() + '/someMethod',
            // note that these are just $.ajax() options
            options = {
                url: url,
                type: 'POST'
            };

        // add any additional options, e.g. a "success" callback or data
        _.extend(options, opts);

        return (this.sync || Backbone.sync).call(this, null, this, options);
    }
});