A couple of my oldest hubs have pretty steady traffic, even though I did next to nothing to promote them until recently. After a site ripens for a while it seems to gain favor with Google if it's relevant to search terms for the topic.
With my newer ones, they get little surges and seem to need frequent rounds of publicizing to keep views... pretty small traffic, so far. I run all my updates through pingler.com, Twitter and Facebook and I'm linking all the hubs on Blogger.
I think one of the keys to maintaining traffic is to write evergreen topics -- subjects people will still want to read about a few years from now. I don't stick to that rule -- lately I did an experiment on a couple of topics that fascinate me. Made me happy that they're reaching readers, but I doubt those hubs will maintain traffic as well as topics for a wider audience do. So far, I haven't had hubs that increase substantially in traffic. They seem to get the most interest right at the beginning and then taper off. Steady traffic adds up over time, though.
I'll look forward to responses from some of the more active Hubbers. I've taken a couple of extended breaks since I started here three years ago. There are many intelligent and helpful people writing here. Welcome to HubPages!