insert this in form:
<p class="forgetmenot"><label for="rememberme"><input name="rememberme" type="checkbox" id="rememberme" value="forever" /> Remember Me</label></p>
In backend, like this:
$credentials['user_login'] = $_POST['log'];
$credentials['user_password'] = $_POST['pwd'];
$credentials['remember'] = $_POST['rememberme'];
..... wp_signon($credentials) ....