You can selectively update certain categories/packages and ignore the other categories/packages. We achieve this by having rsync exclude categories/packages during the emerge --sync step.

By default, rsync will check the contents of /etc/portage/rsync_excludes (if it exists) which contains the categories or packages that you don't want rsync to update.

Note however that this may lead to dependency issues since new, allowed packages might depend on new but excluded packages.

You can ask Portage to use ebuilds that are not officially available through the Portage tree. Create a new directory (for instance /usr/local/portage) in which you store the 3rd-party ebuilds. Use the same directory structure as the official Portage tree!

Then define PORTDIR_OVERLAY in /etc/make.conf and have it point to the previously defined directory. When you use Portage now, it will take those ebuilds into account as well without removing/overwriting those ebuilds the next time you run emerge --sync.

In some cases you want to configure, install and maintain software yourself without having Portage automate the process for you, even though Portage can provide the software titles. Known cases are kernel sources and nvidia drivers. You can configure Portage so it knows that a certain package is manually installed on your system. This process is called injecting and supported by Portage through the /etc/portage/profile/package.provided file.

For instance, if you want to inform Portage about development-sources-2.6.8.1 which you've installed manually, add the following line to /etc/portage/profile/package.provided:

sys-kernel/development-sources-2.6.8.1