There's never enough that I can say about references. They're one of those things not always essential to landing a dream job but you can never tell just when they'll come in handy. Whether they're pre-written or by telephone, here are some points to keep in mind when you embark on reference seeking quest:
Always keep the lines of communication open with your employers. This really only applies if the worked you've worked with has a fantastic rapport with you. Some good signs might be if the employer mentioned that he/she'd write a reference for you before leaving, you left on a good note, or they had to get rid of you due to company wide layoffs. To a degree, this implies a bit of networking on your part - be sure to send a friendly email or card to former employers to keep in touch. Email works best as people tend to move around.
When you need a reference, ask for it. Don't just give your interviewer an email address without preparing the former employer. A courtesy note informing the employer of your intention with an attachment of your latest resume prepares him/her to speak intelligently on your behalf. Instead of being caught off guard by your interviewer, they'll have had time to remember you, reflect on their experience with you as an employee and have some useful notes at hand.
Never forget that the referral is doing YOU a favor. In that courtesy preparation note be sure to ask convenient times for them to be contacted and the best method to do so. Maybe writing the reference ahead of time would be easier so, tell them about the position and why you want it - this is popular among academic advisors and professors. For them, ask formally and politely (usually by letter) for a general reference that you can use later for obtaining a job. Make this process as easy as possible and don't forget a genuine "thank you" for their time.

