used to be /ˈjuːstə biː/ is the correct affirmative spelling for a past state or habit, always with the d on used.
She used to be a teacher, but now she writes full time.
As a child, I used to swim every day. Here used to marks a past habit, and the /d/ ends used before the /t/ of to.
Elizabeth used to be very thin, but she has recently gained a lot of weight. This one covers a past state that is no longer true.
Why the d disappears
In /ˈjuːstə biː/, the /d/ of used and the /t/ of to sit next to each other, so in speech they almost become one consonant sound and the /d/ is not clearly heard.
This is probably because of our ear's influence over how we write: we spell what we hear, and what we hear is closer to /ˈjuːstə/ than /juːzd tuː/. Trust the spelling, with a d, in the affirmative.
A quick way to keep this straight: the form with a d, used to, belongs to affirmative statements like I used to live there. Questions and negatives bring in did and follow different patterns, which this d-blending note does not cover. So when you are writing a plain statement about a past habit or state, the /d/ you barely hear in /ˈjuːstə/ is still there on the page.