<form method="post" action="">
    <input type="text" class="q" name="q" tabindex="1" autocomplete="off" {% if q %}value="{{ q }}"{% endif %}/>
    <input type="submit" value="search" />
    <p>
    {% for category in categories %}
        {{ category }}: <input type="checkbox" name="category_{{ category }}" {% if category in selected_categories %}checked="checked"{% endif %} />
    {% endfor %}
    </p>
</form>