Keep in mind that bringing people to your event and ensuring that they get their money's worth are two different things and need to be addressed as such.
As far as attracting an audience goes, first of all, have something that will attract them. Do this before you really start advertising your event. I see countless ads from everything from comic cons to live music festivals where someone has had a cool idea and started advertising it before booking any bands/speakers/talent. At this point, all you're saying is "Hey, I've booked a hall somewhere! Come and hang out with me!"
So how does one put together a product which will attract a crowd? In short, know your crowd.
What are they interested in? What do they want to know more about? Who are the respected figures in their field? Do they have a broad range of interests or a more specialised area of expertise? Can you perhaps appeal to a wider market and still attract that specialist niche? Answer these questions and book your attractions accordingly. Asking your audience directly is a good (but not foolproof) way of getting this information.
Then, of course, there are the issues of scale and budget. How much are you willing to spend on the event? What is the capacity of your venue? Realistically, what are people willing to pay to attend and how many attendees can you reasonably expect? How many bodies through the door do you need to break even?
Now that you've got something which will hopefully draw a crowd, think about advertising. The web/social networking are inexpensive ways to reach people, but they're also heavily saturated. The amount of crap people have to wade through to get through content leads them to be ad blind, and many people will be using ad blockers anyway, so that's really not the best way to grab their attention. Instead, the best way to bring people to your site is to give them something worth coming for. This could consist of white papers, tutorials, interesting articles on subjects relevant to your audience or well written profiles of speakers. If they Google for a topic and your site comes up, and they find something informative/useful, they're going to be more inclined to attend your event and think that they'll get something of value out of it.
Other means of promoting your event could be direct communication - mail out info packs to companies or individuals you think would be interested in your event - displaying promotional materials at similar events (in my experience many event organisers are willing to do this on a reciprocal basis for the right to poster/flyer your event), advertising in magazines relevant to the field. Be wary of advertising too widely. Linux Übersysadmin Monthly will probably pull a hardcore Linux crowd. Linux Noob Magazine might get you a few interested parties. PC User is much more of a scattergun approach (replace magazine titles/subjects as required.)
Also be aware that many media outlets will be willing to come and go on advertising costs, especially if you provide them with a booth and access to your panels, etc.
OK, so you've brought your crowd out and you're getting all set for the big show. Now you just need to deliver on your promise of the best conference ever!
Make sure everything runs as close to schedule as possible. Some delays might be inevitable, but you can really help yourself out by making sure you have early access to the venue to set up. You might need several days to prepare the venue depending on the scale and nature of your event.
Hire competent people to whom you can delegate responsibilities. Hire decent equipment. You don't need to blow your budget on sound, lights, bells and whistles, but make sure that the gear you have can be relied upon to work consistently over the course of your event.
Make finding information as painless as possible. Produce a programme with a simple schedule of events. If anything changes, announce it on your web site, over the PA system, at the start of other talks and print it out on some sheets of A4 and stick them up around the conference room. There's nothing worse than missing the talk, lecture or panel you came to see because you weren't aware of a schedule change.
Have some side attractions available. You could book a covers band, a few comedians, a guy who juggles poodles. Be creative. If your event is in a hotel this can get your attendees spending more at the bar and incline your venue to give you a better deal next year (and some venues will give you a kickback on bar profits). It also obviously gives your punters some entertainment beyond the conference content.
Finally, make sure that your attendees keep thinking about your event, even after it's over. Give them handouts with lecture content to take away with them (on a USB stick with your logo on it, if affordable). Stationary, mousepads, T shirts and other freebies can help with this.
If your first conference breaks even, you'll have done well. Once you've proven that you're capable of running a well attended, well organised event you'll find that you have a "base" of repeat attendees, making it easier to fill the hall with newcomers through word of mouth and advertising. You'll also find that exhibitors are more readily willing to buy booth space as they know they're likely to get a return on the investment.