One feature I found out about recently is that you can install packages globally rather than locally into your project. I think that this is most useful for development tools, such as PHPUnit which are then available everywhere.
To install a composer package globally, you run the usual require command, but with the addition of the global modifier. So to install PHPUnit, you would run:
$ composer global require 'phpunit/phpunit=3.7.*'
This will install PHPUnit and all its dependencies into the ~/.composer/vendor/ directory and, most importantly, the phpunit CLI tools are installed into ~/.composer/vendor/bin/.
Simply add this directory to your PATH in your ~/.bash_profile (or ~/.bashrc) like this:
and phpunit is now available on your command line.
To keep your tools up to date, you simply do this:
composer global update
To remove a package, you edit ~/.composer/composer.json and then run composer global update.