I have used a device, don't know what it's called, to successfully accomplish this.
Available at most hardware stores. It looks similar to a vibrator. But it's made of rubber, even has a slit at the end. You attach it to the end of your hose, I have a ball cock on the end of mine. Close off the the ball cock, turn on the water, insert into the wall section past the trap. SLOWLY open the ball cock, let it fill up with water thereby sealing, mostly, that pipe, then open the bc all the way releasing the built up pressure through the slit, blasting whatever out.
Something like this, comes in different sizes.
http://www.google.com/products/catalog?hl=en&q=drain+opener&ved=0CCkQzAMwAw&cid=7667024145767101564&sa=title#p