I find it's best to visit a few times so they can see you are a regular, then bring up the Bitcoin topic

Also helps to show them BTCMap and mention the other businesses in the area

Having a BTC user group really helps as you can literally bring a whole bunch of customers to their door (if they will accept BTC as payment, of course)

In addition, go to a print shop and make some flyers, and stickers, for your newly on-boarded merchant(s). Stick them on BTC Map. And write a trip advisor review for them, mentioning bitcoin!