Earn money while waiting to sell

I do a case by case thing, sometimes I park the domain and sometimes I don't. If I have a site that is very active and with a lot of traffic I don't park it. I figure I can wait a bit longer before the domain is sold since it is generating incomes while I wait (adsense and bidvertiser incomes, banner ads, affiliate products etc). OK so the incomes are quite modest on each domain, perhaps $40-$50 per week, but with a portfolio of 20-25 domains it still generates more than $1,000 per month.