What's the negation of "I used to be"? Surely not "I didn't used to be"?

What is the negative form of "I used to be"? I often hear "I didn't used to be" but that sounds awfully wrong in my ears.

The best way* to negate the construction, used to be, is to simply use never and replace the infinitive form of the verb with its past tense. Thus:

• I used to be a chain-smoker / I never was a chain-smoker
• This place used to be a library / This place was never a library
• He used to be such a decent plumber / He never was a decent plumber

It is also correct in some situations to say never used to:

• We used to go there / We never used to go there

In this case, never used to usually serves as an emphatic negation of something previously stated.

Your example, I used to be, is best negated as I never was/I was never, although, I never used to be and I didn't use to be (grates on my ears!) are also options, depending on the context. For instance:

• I never used to be like this!
• Things never used to be this way.

Also consider the following:

• You used to be such a good friend
• I never was a good friend.
• No, I never used to be.

Another similar set of examples:

• You used to fall sick all the time.
• I never used to fall sick all the time.
• I didn't use to fall sick.
• I never fell sick at any time!
• I was never sick.

You should always be able to determine the correct negation to use, as long you keep the definition of used to in mind:

describing an action or state of affairs that was done repeatedly or existed for a period of time in the past

— New Oxford American Dictionary (2nd Edition)

I didn't used to is strictly ungrammatical, though widely used informally or colloquially. (The correct form is didn't use to, although this is also very informal.) Using the past tense of another verb after didn't, in this case used, is grammatically incorrect. Consider this and other similar verb constructions:

• used to be : didn't used to be [Wrong]
• had to have : didn't had to have [Wrong]
• seemed to know : didn't seemed to know [Wrong]

Now, the problem with these examples can be rectified by converting the past tense to the infinitive:

• used to be : didn't use to be [Correct, but not widely accepted]
• had to have : didn't have to have [Correct]
• seemed to know : didn't seem to know [Correct]

Also, did not/didn't always precedes the infinitive form of the verb it helps:

• didn't go [didn't went? — No way!]
• didn't help [didn't helped? — No way!]
• didn't matter [didn't mattered? — No way!]

Here is a note from NOAD to back me up:

Usage

1 The construction used to is standard, but difficulties arise with the formation of negatives and questions. Traditionally, used to behaves as a modal verb, so that questions and negatives are formed without the auxiliary verb do, as in it used not to be like that and used she to come here? In modern English, this question form is now regarded as very formal or awkwardly old-fashioned, and the use with do is broadly accepted as standard, as in did she use to come here? Negative constructions with do, on the other hand (as in it didn't use to be like that), although common, are informal and are not generally accepted. 2 There is sometimes confusion over whether to use the form used to or use to, which has arisen largely because the pronunciation is the same in both cases . Except in negatives and questions, the correct form is used to: we used to go to the movies all the time (not we use to go to the movies). However, in negatives and questions using the auxiliary verb do, the correct form is use to, because the form of the verb required is the infinitive: I didn’t use to like mushrooms (not I didn't used to like mushrooms).

— New Oxford American Dictionary

*(in my opinion, that is)

Inspired by the comments, I have a possible answer for the confusion between didn't used to and didn't use to. In general American, I used to is pronounced with an /s/, while I use or I used in other constructions is pronounced with a /z/. I didn't use to is also pronounced with an /s/, but I believe this may be the only time that /use/ is pronounced with an /s/ and not a /z/, so it is a very understandable mistake to transcribe I didn't use to as I didn't used to. As far as I can tell, the /justə/ (or /justu/) part of I didn't use to and I used to are pronounced exactly alike. Except for not giving the pronunciations, this is pretty much what NOAD says, as quoted in Jimi Oke's answer.

A Google Ngram for American English is very enlightening:

People are now usually spelling didn't use to as didn't used to, and this construction is replacing used not to. For British English, you can do the same thing, and you find that while the construction in the UK is still generally used not to, the ungrammatical spelling didn't used to is replacing the grammatical spelling didn't use to.

This is a surprising development. A pertinent xkcd comic comes to mind. But I am rather drunk now, so I will save myself the trouble of deleting emotional remarks in the morning. For the record, I never typed-and-deleted any such remarks.

This is what the 3rd edition of Fowler's Modern English Usage has to say on used to (my emphasis in bold):

[...] (e)(Only in very informal contexts) With do-support in negative and/or interrogative constructions: He didn't use to wear gloves—P. Cheney, 1964. [...] Prostate cancer ... didn't used to be a problem—Times, 1995.

(f) (Now regarded as somewhat formal) Without do-support in negative and/or interrogative constructions: You usen't to be like that—A. Christie, 1964; [...] I used not to dream—N. Bawden, 1987.

The negative/interrogative type Use(d)n't people to ... is also found, esp. in spoken English and in informal letters, and arguments rage as to whether it is 'better' than the type Didn't people use(d) to ...? Restructuring of the sentence is often the way out. People used to ... didn't they? is perhaps the best way to avoid the problem.

As with so many problems of language, the only satisfying solution seems to lie in recasting the sentence. Writers who want to be read should be prepared to do some work. Opinions differ on the extent to which speech should follow writing, as it can be heard on the street every day.