Well I think there is no best way to encourage offline store to accept BTC. I said this since the transactions now are slow and if more joins in I think the network couldn't handle it. Maybe in the future bitcoin can handle it but not now. For now just let time tell what will happen in the future and let's wait and see if someday many offline stores will accept BTC.
I think the best idea for slow transaction of bitcoin better to release a bitcoin atm so that we can use bitcoin atm card just like a normal credit cards .
So no need to wait for confirmation and i think you can incourage them to accept it..