Keeping the relationship warm for years

Referees have a shelf life. Somebody who managed you four years ago and has heard nothing since will be polite and unspecific, which is the weakest kind of reference there is. A short message once or twice a year — what you are doing now, congratulations on something of theirs — costs nothing and keeps it alive.

And close the loop every single time. Tell them whether you got it. People agree to do this repeatedly for those who let them know how it went, and quietly stop agreeing for those who never do.