Jo Campbell-Amsler gave me a heads-up about Lene Rasmussen's blog Willows: Growing and Working With Willow after a teaching gig in Ontario. Last summer, Lene had a nice post about pruning willow sculpture.
When pinching off the shoots it is important to be careful not to damage the bark on the tree. If the shoot has been left too long you may rip some of the bark off if you "rub" or pull on the shoot - risking that that particular willow rod will die. If the shoots have been allowed to grow too big, it is a good idea to pinch them off with your finger tips or cut them off with a small pruning tool.You'll find the entire post here, along with tips for getting lovely, bushy crowns like the one shown above.