Hi Molly,
I've not fiddled with parking. Instead, I have found decent results generating an income from domain names via affiliate marketing. Basically, I find domain names with existing, qualified traffic and simply forward those domain names to an affiliate marketing landing page provided by the company I've signed up with as an affiliate. See eaasupplements.com and mortgagecashoutrefi.com as two examples.
In short, some people out there bypass Google by typing in keywords that relate to what they are looking for and then tacking on .com just to see what is there. If they like what they see, they buy, and you get a commission as an affiliate. It could be 5% to 40% of the sales price or a fixed, flat fee per sale. Transactions are often few and far between but the payouts are much higher than parking when they do occur.
The key to success with affiliate marketing is two-fold:
1) Finding affiliate marketing companies that will take you on without you having a completely built out website and just a couple of domain names to forward. Many of these companies don't like an affiliate partner simply forwarding domain names as a marketing solution for their brands;
and
2) Finding and buying domain names with existing, qualified traffic and testing out the traffic to see if any of it converts over 12 months. Keep the few domains that generate income (pay for themselves) over 12 months and ruthlessly drop the duds. Over the years, build up a portfolio of 'keepers' that consistently generate income for you and you've got a nice little generator of passive income.
While that sounds great, it can be a lot of risk-taking in paying up for domain names with traffic only to find out they don't convert at the affiliate marketing landing page(s) you forward them to. All you can do is take financial risks in buying, test domains/landing pages and learn what works, keep renewing the domains that pay out, and dump the domains that don't.