I've only been here for about two weeks, so my earnings to date on 6 hubs are about $.86, which is pathetic. I've made $.04 from a single eBay click, which is also pathetic. My best profit is from an Amazon Associates "aStore", which I created specifically for my best earning, most commercially oriented hub (its about survival gear, so it's a pretty smooth connection from hub to store, I think). The aStore has made me almost $3.00, and that's based on a 4% commission on one sale.
Information that may help: that best earning hub gets about equal hits from craigslist and hub pages. I advertise in about six cities with very similar ads including a link to my hub and my aStore directly.
So, in short: the advice everyone gives is good: more quality hubs is the foundation. You won't get rich on 6 poorly written hubs, but you might do OK on 30 or 50 well written ones. While you're at it, shoot for 100. Why not, right?
Also, social networking and outside advertising will help: every time you publish, you are offered the chance to share by Facebook and Twitter. I don't, but I can see that helping. Craigslist has given me a good start (I guess), so I'm sticking with that. I've seen people suggest Xomba (I'm using this in a very limited capacity), Reddit, and other blurb type sites.
Write for other sites that allow back linking, too. Think of it as weaving a net. The bigger the net, and the smaller the holes, the more fish get caught in it.
Finally, comment on other people's hubs. It gets your profile out there; if people like your comments (on their hub or just see you while visiting another persons)they may check out your hubs as well. I chose to use a gimmicky profile pic. I know I have clicked on people just because they had a weird, hot, or otherwise intriguing pic. Seems stupid, I know, but every little bit helps (I actually kind of like my zombie-Ross pic. It grew on me).
I'm no expert, but this seems like a solid approach to me. Good luck!