Very hard!!!!!!!!
I have about 250 domain names, When I first started out they were all listed with Sedo and parked with them. Think I got about $25 the first month.
Then the next few month's the figures really dropped. Then found this site and asked the same question as you.
You have other parked company's to your disposal.
NAMEDRIVE
PARKED
FABULOUS (excuse the spelling, i am sure someone knows the correct spelling!!)
Don't keep your eggs in one basket, spread them out, and see what results you get.
I had 1 .DE WITH Sedo getting 300-400 hits a month, no revenue though, so i get those hits directed to one of my sites!!
Make sure you use the KEYWORD section, at the end of the day KEYWORDS are the most important thing.