Returning errors into templates
I just return the form model object back to the template. Yii has built in validation and error handling within the models themselves. In the example email is the instance of your form's model.

The template:

{% if email is not defined %}
    {% set email = craft.postmaster.email() %}
{% endif %}

{% for error in email.getAllErrors() %}
    &lt;p&gt;{{ error }}&lt;/p&gt;
{% endfor %}

&lt;p&gt;
    &lt;label for="toEmail"&gt;To Email&lt;/label&gt;&lt;br&gt;
    &lt;input type="text" id="toEmail" name="toEmail" value="{{ email.toEmail }}" /&gt;
    {% if email.getError('toEmail') %}
    &lt;label for="toEmail"&gt;{{ email.getError('toEmail') }}&lt;/label&gt;
    {% endif %}
&lt;/p&gt;


The controller:

$model = new Postmaster_EmailFormModel(craft()-&gt;request-&gt;getPost());

if(!$model-&gt;validate())
{ 
    // Prepare a flash error message for the user.
    craft()-&gt;userSession-&gt;setError(Craft::t('The email could not be sent.'));

    craft()-&gt;urlManager-&gt;setRouteVariables(array(
        'email' =&gt; $model
    ));
}
else
{
    $event = new Event($this, array(
        'email' =&gt; $model
    ));

    craft()-&gt;postmaster_forms-&gt;onEmailFormSend($event);

    $this-&gt;redirect(craft()-&gt;request-&gt;getPost('redirect'));
}