Parking is easy but pays the least, and you don't really know what % you get from the middleman.
Even if you do well at parking, you could do better developping, say, an information site about your domain subject, and putting a combination of affiliate programs and adsense on it. Every visitor to the site can be a lead you collect.
Parking your domains is the price you pay while holding your domains and waiting for something better to do with them.
It's good that there is a transparency movement afoot for parking. Parking sites should also start giving the option to add affiliate program links, and some of your own content. They could have some default affiliate programs you could opt into, for example.
There is a big market waiting for the company that can come up with a better parking solution, domainers will flock to it based on all the negative I'm reading lately.