This is a great thread and a challenge that I deal with monthly.
I find that, if you're willing, offering your attendee list is popular with venues in considering free or reduced rates for hire. In addition, giving them the chance to both address and network with your audience at the event is another attractive supplement to your pitch.
Another handy offer, should the event be suitable, could be to invite them to bring their key or prospective clients to your event using it as a tool for them to entertain and/or sell...
To wrap up, I'm a big fan of building relationships and helping others, so I will offer to introduce the venue to any partners/sponsors that are supporting the event - this way you are leaving them with the opportunity to create something new for their future clients.
I look forward to the discussion