The best advice I can offer is to know what kind of information employers are seeking in order to ensure your references are the most effective, in order to accomplish this you can:
Be in contact with direct/indirect superiors for past positions in order to determine how active they're willing to be or act on your behalf.
Explain to your reference what are the qualities or areas of expertise you'd like them to highlight (according to the qualities the employer is looking for).
Provide as much information as possible regarding the position you're aiming for to your reference in order for them to be able to portray you in the best possible way.
