How do I add an extra attribute in my input for Django forms?

Add the attrs keyword argument to your field constructor's widget, and include write your attribute in there:

address = forms.TextField(widget=forms.TextInput(attrs={'placeholder': 'username'}))

If you want to see it in action, take a look at django-registration's forms.py.

Alternatively, you can use the http://pypi.python.org/pypi/django-widget-tweaks app:

{% load widget_tweaks %}
... 
{{ theform.address|attr:"placeholder:username" }}