Hi,
I agree with Colorado and our plan is a similar approach.
We do have several sponsors on the board (and they pay for our running costs), but finding more advertisers is very difficult.
When we started the banner rotation system, I send out 20 emails to barowners with websites offering them 10,000 free impressions on the board. All they needed to do was send me a banner. Two replied.
While my site does with a niche, many potential advertisers do not wish to be associated with a site which also deals with nightlife / prostitution. And those in the business themselves don't bother advertising since they either don't see the point, can't afford it, or don't need to.
However, since we are a niche site (and arguable our site / board is the best in this niche) we have a strong following - a following that raised $4000 last year for us to buy a faster server! - and we feel strongly that by offering them a host of new features, at a fee, that many will take the offer.
By doing this revenue will come from both advertisers / sponsors and members, allowing for a (hopefully

) greatly increased amount, which in turn means that we can spend more money to improve the experience for all members (better hardware, really nice prices for competitions, etc).
We are also looking into offering discount cards to our members, to be used in bars, restaurants, hotels, etc. This however is still in a very early stage, but could also generate some additional income as well as provide an additional incentive to sign up.
Sanuk!
PS Were you refering to my site when mentioning 'top 3 from a revenue standpoint'? If so, we might be, I have not really looked at many other sites.