The big players are not too concerned with parking income if they are selling domains. The profit from that takes the pressure off. Of course if they also get some good money from parking while they are waiting, that's great. But generally you can't just park and then expect to earn much unless you get type-in traffic from you domain.
Check with your domain parking company and see if you can put up links to your parked domains. Some allow this and others don't. If you don't have a domain that will get type-in traffic, you need to find a way to generate some traffic to the domain that does not involve fraud or violates their Terms of Service. You can try to get links from other sites and diferctories, but most are not interested in linking to parking pages. A good idea then is to have sites and blogs, and put the links on your own sites.
Use a parking service that makes great parking pages, or that allows you to optimize them. If you optimize the pages, do some keyword research to find out what the keywords are with high bid prices. If you have a domain with a topic that is mostly low bid keywords, try to find something related with higher prices. For example, on a computer game site you could have some computer game console keywords that will pay you more.
Try several parking services. Spend the time to optimize the pages, then wait for a month or two. If you don't make anything, move the domain to another provider and try again.
In general, most sites that are pretty good will earn more money than parked pages for domains that don't get type-in traffic, but it does depend a lot on the bid prices.
