<action method="disableParamsMemorizing" />

public function getToolbarBlock()
{
    if ($blockName = $this->getToolbarBlockName()) {
        if ($block = $this->getLayout()->getBlock($blockName)) {
            $block->disableParamsMemorizing();
            return $block;
        }
    }
    $block = $this->getLayout()->createBlock($this->_defaultToolbarBlock, microtime());
    $block->disableParamsMemorizing();
    return $block;
}

Go to System->Configuration->Catalog->Frontend
select from 'Product Listing Sort by'

appcodecoreMageCatalogBlockProductListToolbar.php

protected $_direction           = 'asc';

protected $_direction           = 'desc';