I have used a magnet and a dress sock before. Simply add water to the bottom of the keg, put the magnet in the sock and the drop in. Swirl the keg around and 90% of the metal shaving collect on the sock/magnet. Remove sock and then rinse and wash.
Hope this help
Madbier's Malt House
On Tap: Hopped up Cream, Madbiers Summer Pale, Madbiers Weizenbock
Kegged: Madbiers Red Ale
Primary: Madbiers Weizen