Best way to install packages for TeXLive in Ubuntu?

Ubuntu and Debian ship with a totally outdated TeXLive 2009. The current version is 2011, released a couple of weeks ago. I highly recommend installing TeXLive using its own installer instead as described in How to install “vanilla” TeXLive on Debian or Ubuntu?. Then you can install packages using the normal tlmgr package manager:

tlmgr install <package name>

To update a package use:

tlmgr update <package name>

To update all packages (and tlmgr itself):

tlmgr update --self --all

This tool is not shipped in Ubuntu because they don't want any other tool installing files. Unfortunately software like the LaTeX editor Kile has TeXLive (the Ubuntu version) as its dependency, even if there is no binary one. They simplest way would be to keep the old TeXLive around and not to uninstall it. You might be able simply delete the files, though.

A lot of frequently used *TeX packages have been combined into Ubuntu packages. In this case a quick search in the Ubuntu Package Manager shows that xlop is part of the texlive-generic-extra package, which you can install using apt-get install or the GUI as preferred.

Having said that, it's not particularly difficult to install *TeX packages manually once you've identified the right tree to put them in (CTAN has pre-zipped versions of a lot of packages, which makes it even easier - you just have to unzip them at the right place and run texhash), and you then have the advantage of being sure you're up-to-date. At the moment I believe the Ubuntu versions are lagging quite a way behind TeX Live releases (let alone any subsequent package updates).