class MyForm(forms.Form):
myfield = forms.CharField(widget=forms.TextInput(attrs={'class': 'myfieldclass'}))
Or
<form action="/contact/" method="post">
{{ form.non_field_errors }}
<div class="fieldWrapper">
{{ form.subject.errors }}
{{ form.subject.label_tag }}
{{ form.subject }}
<span class="helptext">{{ form.subject.help_text }}</span>
</div>
</form>