django form resubmitted upon refresh

After I submit the form for the first time and then refresh the form it gets resubmitted and and I don't want that.

Here's my form in template :

<form action = "" method = "POST"> {% csrf_token %}
    {{ form.as_p }}
    <input type = "submit" value = "Shout!"/>
</form>

How can I fix this ?

Here's my views:

def index(request):
    shouts = Shout.objects.all()

    if request.method == "POST":
        form = GuestBookForm(request.POST)
        if form.is_valid():
            cd = form.cleaned_data
            shout = Shout(author = cd['author'], message = cd['message'])
            shout.save()
            form = GuestBookForm()
    else:
        form = GuestBookForm()

    return render_to_response('guestbook/index.html', {'shouts' : shouts,
                                             'form' : form },
                              context_instance = RequestContext(request))