1) Don't rely on advertising to monetize 2) Don't target it to consumers/end-users (if you're selling a web service, obviously this does not apply if you are selling an actual product)
There are many B2B niches right now that have terrible software solutions people are willing to pay for improvements to.
For example: About a year ago, I created a very niche web service for Internet marketers. It was basically a PayPal subscription tacked on to a few custom scripts I wrote to automate certain marketing tasks for myself. I didn't advertise it at all except for posting on a couple of Internet marketing forums I frequented. The site only had a few hundred visitors a month, however about 10% of those converted and became subscribers. Within a couple months, I had about 30 subscribers paying $50 a month to use the service.After I wrote the code for myself, the site required no work to maintain, the subscriptions were coming in on autopilot, it was an easy $1500 a month with no effort.
I was surprised at how easy it was to make a little extra money this way- the key was focusing on a very specific, small B2B niche, and creating a paid subscription service that was far superior to anything else that was available.