A trick I learned from Ramona Schultz, a Toledo Monokote winner, is to constantly shrink well ahead of the area you're ironing down. That keeps you from suddenly having a whole lot of shrinking to do at once. Sometimes you'll only iron down 1/16", it just takes time.
Ditto to MinnFlyer's remark on having plenty of extra covering to hold on to while you're stretching. Covering is expensive, but that's no place to economize.
As an aside, Ultracote is much easier to use on wingtios because you can actually pull it down over the tip to stretch it without seperating the adhesive and the color from the film, but Ultracote has its own negative features, too.