I read through all the students' posts. I reply when I am "inspired" to post. That means that sometimes I read a post and nothing immediately comes to mind to respond. Then for other posts I immediately think of something I want to share in response, so I post that. My overall goal is to give the students the sense that I am actively reading and engaging in their postings as well.
Some weeks I start by reading and replying to any posts that no one else has replied so they won't feel like no one cares about what they had to say.
I'd probably do more of what Vardahilwen does if I had more students (I have 14 students in my online course).
Chime to all of the above.
I also try to keep a mental note of who I have and haven't replied to recently, and try to even things out. From a student point of view, it seems really weird if the instructor never replies to your posts! Also, keep a close eye out for students who are asking you a question or otherwise looking for clarification.
For our very first discussion, which is just a "hello" sort of thing, I make a point to reply to every single student quite promptly in order to welcome them to the class.
Beware LarryC's approach. Though it may have some truth to it, you need to then find ways to establish your online presence elsewhere. (I've known instructors who use that as an excuse to not reply to any discussions ever. Not good. Just plain lazy).