You can make a call to number 411 to unlock Brain-in-a-jar hat, while to unlock police hat and gun belt you will have to call 911.
The codes/phone numbers for unlocking Santa hat & sack, and Nerd costume Ned Noodlehead you will have to call numbers 1225 and 1337, respectively.
However, you must keep it in mind that getting these secret clothing items will write over whatever you are wearing, for example, whenever you change your outfit using “costumizer”. What is more, these secret clothing items can not be straightforwardly removed!
There are also some keyboard combinations that can be used to make some changes:
You can change your character’s color by pressing three keys, viz. CTRL + SHIFT + S at the same time; while you can change the color of your hair by pressing CTRL + SHIFT + H.
Pressing CTRL + SHIFT + P will give you a pumpkin head, and pressing CTRL + SHIFT + R will allow you to randomize your character.