I've done this twice - once with great success, once not. In my case it was all down to the pub -the choice of pub is crucial. It's a good excuse for a pub-crawl with a mate or two, to explore the possibilities of the local hostelries. The best set-up, I'd say, is a pub with one recognisable section that can be earmarked for the session - ideally not totally separate from the rest of the pub, so that other punters can hang around and listen if they want to. It's also important to understand the boundaries (if any) between the session players and the people who have come in for a meal - sometimes they put the musicians in the restaurant area without keeping tables aside for them, which can cause problems for musicians and unsuspecting diners alike.

And then you need to get it across to the landlord that you are not offering to set up a band like the Pogues in the corner of the pub for free. Some don't really understand what an open session entails. Once it's clear, you may be able to get him to provide a free drink or food for players.

And, as others have said, do try to persuade a few tried and trusted musicians to come with you for the first few sessions at least, to make sure there's a core of players to get things going. Be welcoming to newcomers and friendly to listeners-in.
Good luck!