Wah How to use "Used to"

We use "Used to + v." to refer something that happened regularly in the past but does not happen now. 

Affirmative           
     You used to visit Wah Wasabi's website every day but now you never visit it again. (T__T - Feeling sad)
     I used to be an adventurer like you until I took an arrow in the knee. (FUS RO DAH !!!)

Negative
     I didn't used to smoke. is incorrect. 
     I didn't use to smoke. (I didn't smoke in the past but I smoke now.) 
     I never used to smoke. 

Interrogative
     Did you used to play soccer? is incorrect. 
     Did you use to play soccer? 
     Did he use to smoke?

We can also use used to describe the state of something...
     There used to be a library here but now there is a hospital instead.
     There didn't use to be a hospital here (There used to be a library here.)

Used to do' is different from 'be used to + doing' and 'get used to + doing' that we will mention later.

Gotye - Somebody That I Used To Know (feat. Kimbra)
   "But you didn't have to cut me off
Make out like it never happened and that we were nothing
And I don't even need your love
But you treat me like a stranger and that feels so rough
No you didn't have to stoop so low
Have your friends collect your records and then change your number
I guess that I don't need that though
Now you're just somebody that I used to know..."