Set Hidden

If you've ever tried to type :only and had Vim refuse to hide existing buffers because they contain changes, you can use :set hidden to override this behavior. Just remember to keep the possibility of hidden modified buffers in mind when you're doing a :q!.

Setting the Filetype

Vim normally sets the current filetype automatically and very accurately at that. In some cases though, such as when you're starting a file from scratch, it doesn't know the filetype. In these cases, you can set the filetype manually.

:set filetype=perl
:set filetype=php
:set filetype=python

etc...

Scrollbind

You can :set scrollbind in any windows in Vim that you would like to scroll together.

Thanks to Chris for the tip.