Why do you want people to blog about your conference?
Because you put on good conferences and word-of-mouth is one of the best marketing tools out there. If a blogger brings attention to your event, you are reaching an audience you would not have otherwise reached. And if your conference sounds good, they'll be more likely to attend next year.
What if the blogger hated the event? Chances are, other attendees did, too. Be glad you have someone speaking up about it and use it as an opportunity to publicly address the shortcomings of the event and to discuss how you will reformat it the next time around. Take the opportunity to engage the attendees who were there, as well as potential attendees next time around, to build the next event for them (do this even if the bloggers loved you event).
How do you get people to blog at your conference?
Some attendees are more likely to blog than others. I suspect BlogHer has more live-blogging attendees than, say, the National Association of REALTORS Convention, but you can still get bloggers to your conference. If those real estate professionals have blogs related to their work, why wouldn't they want to take a learning and networking opportunity and turn it into a marketing opportunity by blogging to their clients and potential clients about it?
1. Make it easy for them. Give them free wireless internet access and advertise it. As Mashable points out, this can be a costly add-on for event planners, so leverage it if you have it. (Hint to attendees wanting to live blog: if you want wireless, ask for it. Event planners might not be getting it because they don't think the demand is there. If it becomes a factor in whether people attend or not, they'll add it.)
2. Identify bloggers among your potential attendee pool and invite them. Give them a discount on tuition. Get them in the seats. Just make sure that if a discount is given, they acknowledge it in the blog and that they know they are free to blog whatever they want--even the negative. Transparency is important to you and will be to them, as well.
3. Your speakers know bloggers in their field. Have them reach out to individuals they know.
4. Live blog your conference on an event blog. I hesitated to add this because I don't think it is anywhere near as powerful as having true bloggers in the room, but I think it can serve a purpose. If you have a blog set up for your event, keep it going throughout the event. Highlight sessions and networking events as they happen. Take pictures and get interviews with attendees.
5. Next time you're at a conference (as an attendee), live blog it. This will help you identify the pitfalls and issues so you can better relate and cater to bloggers. If you don't already have a blog, you may want to try an easy service like posterous to get something up quickly and easily.