Answer: Willows are very hardy, and in general enjoy moist soil. Yours will probably recover very quickly given TLC of regular moisture and a good cover of mulch.
Pruning will encourage new growth and your tree will fill out more and more each time it's pruned. If your willow is still struggling, though, don't prune it this year, and allow it to gain strength. Pussywillows develop their showy catkins on new wood and therefore should be pruned in late spring, after the catkins fade. This gives the plant all summer to develop new flowering wood. You'll find your willow to be a vigorous grower that will need annual pruning to keep it under control, and to help it perform at their best.
Wait until after the catkins appear, then prune back any wood that has bloomed. You may want to remove misplaced branches at the same time, to achieve the height and shape you desire.
For complete pruning guidelines, you may want to consult the book 'Pruning', by Christopher Brickell (ISBN# 0-671-65841-7). It has lots of line-drawings to help you through the pruning process.
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