Counting unique values conditionally using SSRS

Sometimes you want to count unique values based on filtered conditions. For example, count the number accounts and contacts in which you have a qualified opportunity.  You would be running the query on the opportunity and the joining back to contact and then account. To do this:

countdistinct(iif(<parameter that makes opportunity qualified>,<field with distinct value you want to count>, nothing))

Hope this helps someone out there.