A few quick bits of advice;
1. Find a supportive venue, one which will allow you to put up posters and flyers around the place, if they'll help you sell tickets so much the better.
2. Put the gig on in a separate room, not the main part of the pub and restrict entrance to people who specifically want to watch the comedy.
3. Charge an entrance fee, even if it's a nominal amount (£2 or £3), this does two things, firstly it gives you a budget to pay a headline act who'll hopefully entice people to stay until the end of the show and secondly, if the audience have actually invested in the night they are more likely to stay AND behave themselves a bit better.
4. Make sure you have a decent PA. The audience need to be able to hear the comedians clearly. You can be the best stand up in the world but if they can't hear you they won't laugh.
5. Make sure the lighting is okay. A lot of stand up comedy is visual, a comic's facial movements or body language can make a big difference to the experience, the audience need to be able to see this. Also make sure the stage is well lit but the audience are in relative darkness, audiences relax more if they are hidden slightly in the shadows, nothing kills laughter more than bright light on the audience.
6. Advertise as much as possible, it is impossible to promote a gig too much, acts are easy to find, audience members are not.
7. Try to get a decent compere. An experienced compere can make all the difference to a comedy night, an inexperienced MC with a procession of inexperienced open spots is a recipe for disaster. A really good MC can hold the night together even if the other acts are bombing.
8. Try to be discerning with your booking. I know that this might be anathema to the ethos of an open mic night but try to balance giving everyone a chance with maintaining the quality of the night, after all you need to entertain an audience and try to ensure that they come back again.
9. Don't allow the night to drag on, try to be quite harsh with timings, if people are doing a 5-7 minute spot make sure they know that at 6 mins they should be preparing to get off stage no matter how much they are enjoying it. Have a system where you can flash/wave a light (red bike lights work well for this) when acts should be wrapping up. Audiences get fidgety and need to get home, you might not have work the next day or babysitters to get back to but the audience probably do.
10. Have several intervals. This allows the audience to stay refreshed and means that they won't be coming and going too much during the acts. try to keep sections of the show to 40-45mins, any longer than that and you'll have people getting up to go to the toilet or the bar.
Anyway, that's a few pointers, if you need any more advice or tips feel free to get in touch... oh and if you need an experienced MC or decent closing act I'm based in Wiltshire and relatively cheap!