Changing PHP code which uses PHPs $_REQUEST super global

Question

New to php and have read up on $_REQUEST being strongly discouraged, due to the fact that it makes it nearly impossible to determine the source of the data, and also being a security risk as well.

Not sure what the best way would be to change the following code, to update the code, so that the $_REQUEST is replaced with something better?

public static function sales_data_postback() {
    if ( ! isset( $_REQUEST['sales_data'] ) )
        return;

    $data          = json_decode( stripslashes( $_POST['data'] ) );
    $cart_contents = json_decode( stripslashes( $_POST['cart_contents'] ) );

    //Unset purchase log ID, since we're inserting a new one.
    $data = (array) $data;

    unset( $data['id'] );

    $purchase_log = wpsc_get_order( $data );
    $purchase_log->save();
    $purchase_log_id = $purchase_log->get( 'id' );

    global $wpdb;

    //We need to update the proper product ID, name and purchase ID
    foreach ( $cart_contents as $cart_item ) {

        $product = new WP_Query( array( 'post_type' => 'wpsc-product', 'pagename' => $cart_item->slug ) );
        $product = $product->get_posts();
        $product = $product[0];

        $cart_item = ( array ) $cart_item;

        unset( $cart_item['id'] );
        unset( $cart_item['slug'] );

        $cart_item['prodid']     = $product->ID;
        $cart_item['name']       = $product->post_title;
        $cart_item['purchaseid'] = $purchase_log_id;

        $wpdb->insert( WPSC_TABLE_CART_CONTENTS, $cart_item );
    }

    die;
}