Your most powerful reference is the one who calls an employer before the employer calls him. A preemptive reference speaks up for you, not about you. Actually, this is not a reference at all, but a recommendation or a referral.
The traditional reference answers questions about you when asked. This makes for a dicey situation because he or she is probably unprepared to talk about you on the spur of the moment.
A preemptive reference thinks enough of you to pick up the phone and call the manager who’s considering hiring you and recommends you without being asked. This is a big step beyond a reference; it’s a true professional courtesy.
The best preemptive reference is when a reputable person in your field refers you to another employer. In other words, the referrer sends you — they used to say transmits you — to a peer and asks them to hire you. Who needs a resume when you’ve got that? This is beyond even a professional courtesy; it’s an endorsement. It carries enormous weight.
When someone you respect asks you to be their reference, take it a step farther. Start a trend among your peers: provide a preemptive reference. Maybe someday someone will do it for you.
(Worried about bad references? See How can you fight bad references?)