Quickform: Set the "Selected" value in a select element
This is how you do it:

CRM_Financial_BAO_FinancialType::getAvailableFinancialTypes($financialTypes, 3);
    $this-&gt;add('select', 'financial_type', ts('Financial Type'), 
            $financialTypes, 
            'TRUE'
            );
    $this-&gt;_elements[4]-&gt;setSelected(9);


You need to inspect $this and determine which of the elements is the select you want to modify. In my case it was the 4th one. 

You then need to inspect the select (from the browser) and find out the value for the option that you want selected by default.