during my basic aerobatics course on a De Havilland Chipmunk, I was taught to pick a prominent feature on the horizon on either wingtip, i.e. 90deg of current heading, then lower the nose into a shollow dive to gain speed (possibly around 120mph), and then simulataneously pitch up and roll steadily until the "feature" previously selected appears directly on the nose as the aircraft passes thru inverted straight and level position, now without any extra rolling or pitching forces it was just a matter of allowing the manouvre to "complete itself".