In addition to tweaking keywords you can rotate the templates used on your domains to see which convert your traffic best. With all my new clients I run their names through a template rotation schedule ideally devoting one week per template for at least one month to see what performs best. Talk to your account representative at whatever parking company you use and they should be able to help you determine which templates would be the best to test for certain names.
You may also want to structure the revenue generating arm of your portfolio to include retail sales, leased traffic, and developed sites in addition to parking revenue.
Chat with your account representative about your names to see if you should use these tactics with some of your current names. A lot of times there are programs at the parking company that will help you with much of this in addition to basic optimization.