If you run a forum (or if you're thinking about setting one up) then you can make users pay for access to it, or just pay for access to specific areas.

A good example of this in action is with the SEObook forum or the WebmasterWorld forum.

Advantages: If your blog community look at you as an expert within your niche then your advice can come at a premium - with premium content areas you have a huge potential for making long-term revenue.

If you offer a monthly/weekly/yearly retainer membership then you can secure long-term income from your members. Not only this, but you'll also have loads of opportunities to up-sell and remarket to your premium members to make further revenue streams.

Disadvantages: By creating content barriers to your readers, you may risk losing readership.

Unless you're going to offer something of genuine value to your members, I'd recommend not going down this route. Remember that the web is a big place, so if someone can get information from elsewhere easier then they probably will.