Unfortunately we have had a couple of incidents in Mono County over the last week, involving hot ashes catching structures on fire.
To dispose properly of ashes, I use a metal can that I purchased at Home Lumber Company in Bishop. Every 7-10 days our wood stove needs to be cleaned out. I scoop the ashes from the stove and place them in the metal can, where I let them rest covered for 7-10 days. Once they are rested and I am assured the are no longer live, I then place them into our garbage can. I set the can outside until the next time I need it.
The picture below is of my 8 mo old son. He loves the ash shovel... if I don't hide it, the shovel becomes his toy. Inside the stove are some coals from the mornings burn. Ashes can easily last 24-48 hours in a stove or metal can, so you might as well just let the ashes sit in the metal can for as long as possible.
In summary, use a metal can and let them sit in the can for a week before disposing of them into your trash can. Even better yet, use the ash in your garden soil.