Some examples:
"Kuroshitsuji"/"Black Butler"
"An No Exorcist/"Blue Exorcist"

I always refer to them by their main name I only use Kuroshitsuji,but in terms of a name like "Ore no Imouto ga Konna ni Kawaii Wake ga Nai" then I just use its say "nickname" Oreimo for shorter preference to the anime.

I use the english trans for any series and if its a name like The World God Only Knows, I will shorten it.

I like saying: Kuroshitsuji and Black Butler....

If it's easier to say and pronounce.

I try to call an anime by its English name if at all possible. Sometimes a Japanese name will get stuck in my head and I find it tough to switch. Generally, I dislike long Japanese titles, especially if I don't understand it. If I know how it translates though, I might still refer to it as its original name. In the case of "Baka no Test", I find the English translation "Baka and Test" to be weird, and if they aren't going to bother fully translate the name, I'd just as soon use the full original Japanese name.

Well I sometimes use nicknames but never English names.

Yes

I call them by their main name because i am usually able to pronounce it correctly. It is pretty easy for me! :)

I sometimes come up with a random nickname for the anime.

it depends if i hear about the anime and they use the english name first or vice versa

Like~
Suzumiya Haruhi no Yuutsu- The Melancholy of Haruhi Suzumiya- TMoHS.
Ouran High School Host Club- OHSHC.
Angel Beats!- AB!
Most of them are initials! XD