Sheldon Brown has a wheel building/truing article in which the procedure in a nutshell and your vernacular is to work on whichever condition is worse...If the fishtail is worse then work on that first, then when the porpoising is worse work on that. In between you stress-relief (pinch the spokes, flex the wheel, or whichever method you use) as this will often change fishtail and porpoise. You sort of sneak up on a true wheel, and there are some different techniques in the article when you get close to true.