there is no default option in wordpress to set up sort order of post manually while adding post. Other options are sorting by title or date using query_post() function. To set up custom sort order we can do this using custom field. Use following code
query_posts(orderby=meta_value&meta_key=sort_order&order=ASC’);
REMEMBER: if you use this function then you have to assign sort_order custom field to each post. Otherwise posts which do not have sort_order custom field will not be displayed.