How to: change the layout of my cart?

Like for the previous CSS issue, pleas post a link to your website (with an access if the website is in maintenance) so that we can have a look at it.

You need to add the CSS:

.hikashop_payment_methods{ clear:both;}
.hikashop_checkout_coupon{ clear:both;float:left;width:100% }

float: left;

.hikashop_checkout_address_right_part {
width: 350px;
float: left;
}

Also, I invite you to use Firebug. It's a nice addon for Firefox which helps tweak CSS code:
www.tutorial9.net/web-tutorials/quick-ea...opment-with-firebug/