Arch:
An official pacman-package is available. Install the package as root, using:
pacman -S postgis
OpenSuse:
In order to use the openSuse repositories for geospatial applications, enable the Geo-repository as root:
zypper ar http://download.opensuse.org/repositories/Application:/Geo/openSUSE_[RELEASE]/ GEO
where [RELEASE] is the official release number of your Suse-distribution. After this, you can install postgis with:
zypper install postgis