Reference checks - Advice to make them more meaningful?

I've been asked to be a referee for two former employees in the past few days - one I was thrilled to provide endorsement for, and one who I felt I did not know well enough (we'd only worked together for a few months, now more than 2 years ago) so had to politely decline. The recruiter asked me some stock standard questions about the candidate's history which got me thinking; what advice is there to make these more meaningful? What are the most important questions to ask; what weight should you put on them?