I switch between enough different projects with different coding styles that I have shortcuts to change between them:

    set expandtab
    set shiftwidth=4
    set softtabstop=4
    function Spaces(...)
        if a:0 == 1
            let l:width = a:1
        else
            let l:width = 4
        endif
        setlocal expandtab
        let &l:shiftwidth = l:width
        let &l:softtabstop = l:width
    endfunction
    command! T setlocal noexpandtab shiftwidth=8 softtabstop=0
    command! -nargs=? S call Spaces(<args>)
    autocmd BufNewFile,BufRead ~/src/linux/* T
    autocmd BufNewFile,BufRead ~/src/git/* T
    autocmd FileType html S 2
    autocmd FileType tex S 2

This sets the default to 4-space indents, but lets me do :T to switch to tabs (and does so automatically for Linux and Git which use that style), or :S N to switch to N-space indents (and automatically switches HTML and TeX to 2-space indents).