I have the following in my vimrc:
This allows me to take advantage of the convenience of the tab key when editing while actually inserting spaces into the current buffer. Intuitively I've always wished I could shift-tab to unindent the current line one tab stop. After a minute of fiddling with my vimrc, I found a solution.
imap <S-Tab> <C-o><<
I should also mention that << and >> shifts the provided text based on your shiftwidth setting.