Both steam and running under water work very well. I usually start as Susie suggested - by running under water (just because it's fast and easy) but I do it while the soap is still in the silicone mold and before I've pulled the sides away. This way I get water only on the tops and I don't have to worry about sticky sides. Normally I run the loaf under the tap, rub my fingers across the top to remove the ash, then prop it at an angle so the water runs down and I blot it off. Then I let it sit for about 30 min to dry before I unmold. If the ash appears after I unmold, I use a steamer. You can either hold the bar over a pot of boiling water using a mit (to protect your hand from the steam heat) or use a commercial steamer that's made for removing wrinkles on clothing. Both work very well so it's just personal preference on what works for you.
PS - Sorry, I don't have any before and after pics to share.