1. sudo apt-get install ctags (Debian/Ubuntu Linux), or brew install ctags (OS X).
  2. In ~/.vimrc, add set tags=tags
  3. Go to your project directory, and run ctags -R
  4. When editing, put your cursor over a variable, method or class and hit Ctrl-] to jump to its definition.

(Bonus step: Go back to secretly coveting Intellisense.)