Diversifying by utilizing multiple platforms is the best way I've found results (ie. utilizing all 3 mentioned methods)
Using Ad Networks that have not grown past their own seams is ideal for a lower CPC. For example some major keywords on larger networks are upwards of a $1/click where on smaller networks where the demand is lower can be as low as $0.05/click. Essentially the people viewing the ads are not much different with little effort and you get 20 times the traffic for the same budget... In these tough economic times, what makes more sense?