Bind function to key and command

command! YoFunctionHere call s:YoFunctionHere()
nnoremap <silent> x :call <SID>FunctionHere()<CR>
function! s:FunctionHere()
endfunction

Call a function in insert mode

inoremap X <C-R>=script#myfunction()<CR>
inoremap <F2> <C-R>=MyVimFunc()?'':''<CR>

Checking plugins

if globpath(&rtp, "plugin/commentary.vim") != ""

Autoload

" autoload/hello.vim
if exists("g:hello_loaded") | finish | endif
let g:hello_loaded=1

function hello#method()
endfunction

" calling hello#method() will load only if autoload()

Misc

Version check

if version < 704
  echom "requires vim >= 7.4"
endif