Tie the ivy loosely to the wire frame with cloth or wire ties when you first create the topiary. This will train the ivy to grow along the frame. Remove the ties once the ivy has adhered to the frame.
Pinch ivy back with your fingers to 1/4-inch above an outward-growing leaf. This will encourage the ivy to form new stems that will fill in sparse areas in the topiary.
Snip straggling or overgrown pieces of ivy back to an outward-growing leaf or bud to keep the plant looking neat and tidy. Prune a bit at a time, standing back, after a few cuts, to observe the ivy from a distance. Taking time to analyze your work will ensure that you don't prune too much or unevenly.