I am very new to Hubpages but I think I am having fair success after experimenting with a few tricks.
1) This is probably most important because I think it is often overlooked over its simplicity. I think you have to make a very intricate hub which is littered with multiple ads placed all over your hub. I think this is working well for me and I am trying to make my new hubs more intricate.
2) I think other than having obvious SEO, social networking is critical. Twitter will bring people who don't even know what Hubpages is to your article. If you use an interesting hash tag in your tweet, then you will have a very narrow range audience who most likely is looking for exactly what you are writing about! Because of this, I think having a twitter to promote your hubs is critical if you want better CPM as long as the ads are about exactly what you are writing about
3) Obviously a total no brainer, if you really want to increase your CPM just advertise a high quality, low cost item. They may just bid the item up if it is on ebay, or they may just buy something else! Unfortunately I am from CT, and Amazon does not allow me to participate in their program :(