My formula so far (without spaces here):
AND( OR( ISPICKVAL( Type, "Prospect"), ISPICKVAL( Type, "Is this a Prospect or User?"), ) , ISPICKVAL( Client_Management__c,"") )
The other Types of accounts are either clients or ex-clients.
But for the Types specified, I'd like for the Client Management field to be required to be blank (it is a picklist field by the way).