Magento 1.9.1.0 – How to Change Cart URL to Direct to OnePage Checkout

You won't be able to do it natively.

But you could be clever and use web sockets (if you need 2 way comms) to continuously save the form data (just like this website does!) - or just use a vanilla Ajax call onkeyup - then save it in the PHP session, to be echo'ed out later.

Or a more straightfoward pluggable approach would be to use a library like jQuery Save as you Type

In either situation, it would save the form contents and allow them to visit another page and come back. The only thing you might have to do is retain the accordion position - but I'm not sure many customers would mind hitting "next" a couple of times if their data was already saved.

Try and add the following code to

app/design/frontend/YOURPACKAGE/YOURTHEME/layout/local.xml

<checkout_onepage_index translate="label">
    <reference name="right">
        <block type="checkout/cart_sidebar" name="cart_sidebar" template="checkout/cart/sidebar.phtml" before="-">
            <action method="addItemRender"><type>simple</type><block>checkout/cart_item_renderer</block><template>checkout/cart/sidebar/default.phtml</template></action>
            <action method="addItemRender"><type>grouped</type><block>checkout/cart_item_renderer_grouped</block><template>checkout/cart/sidebar/default.phtml</template></action>
            <action method="addItemRender"><type>configurable</type><block>checkout/cart_item_renderer_configurable</block><template>checkout/cart/sidebar/default.phtml</template></action>
            <block type="core/text_list" name="cart_sidebar.extra_actions" as="extra_actions" translate="label" module="checkout">
                <label>Shopping Cart Sidebar Extra Actions</label>
            </block>
        </block>
    </reference>
</checkout_onepage_index>

Put it just above the closing </layout> tag. This should display the minicart in the right sidebar above the progress field, like in the screen shot below.