The Ubuntu Way
The various LaTeX packages, i.e. graphicx, pslatex and so on, are managed by the regular sofware repositories. So you can install must, but not all tex packages with Software Center or Synaptic. However not all LaTeX packages there are up to date and even worse: Not all exist in the repositories. Some of the more related packages are bundled together in one. E.g the Ubuntu package texlive-science includes these latex packages (as of this writing). Search the description of the Ubuntu packages for the latex package you want and hope you find it in the right version.
If you have a lot of free space you can just install texlive-full, which most likely will include all the latex-packages you will ever need and many many many more.
Using texlive and
You can install the latest version of texlive using the net install script of tug.org. You should uninstall the Ubuntu version of texlive before hand. Then you can install packages using the normal
tlmgr Texlive package ManaGR:
tlgmr install <package name>
To update a package use:
tlgmr update <package name>
To update all packages (and tlmgr itself):
tlmgr update --self --all
If you want the latest version or a package not included by both the Software Center and even
tlmgr you will have to install the package manually. Here it depends on the package. Just follow the installation instructions of the packages if it provides any. CTAN and the Tex online catalogue have prezipped versions of many tex packages. Here is a Guide on how to install them.