I high-demand server with a pretty good amount of content of two 1.2GB drivers in a RAID-1 mirror. We're talking web services, e-mail services, web-mail, and a few client sites. I'm not hosting 400MB movies, or MP3s, or anything along those lines. About 12 websites.
One thing I did to free up space was to remove the contents of /usr/share/doc (and other, I forget it offhand, perhaps /usr/doc). While some people may actually have the free time and desire to thumb through the pages of ReadMe's and Squid documentation, I often use the authors website or my best friend, Google, for documentation. By removing these directories I was able to free about 100 MB.
Additionally, I see no value in keeping the rotated log files because I constantly review my logs. If you use your server as a web-server and handle a large amount of requests you can quickly eat up 100MB in logs easily by the time they rotate. I'd remove those rotated logs.
Clearing out /tmp is also a good idea.
Also, I remove any RPM's that I am not using, such as Squid, IMP, Horde, etc. If you are using Squid, you may want to look at clearing out your cache or adjusting your cache settings to something that is more disk friendly.
I had a large amount of RAM on my system, 256MB, and was only using about 64MB (With samba stopped, Samba loves to cache as much available RAM as possible). I never did any kind of disk swap, and despite the taboo associated with not having swap space I've never had an issue for over a two years. I destroyed the Swap partition (about 200 MB), created a RAID partition on it, formatted it, and mounted it to /home/e-smith/files/ibays using /etc/fstab. Since my users don't really use up that much space in ibays, it was an ideal configuration, allocating about 200MB of other-wise unused space. Some people say swap is absolutely necessary, others say it is only necessary if you don't have the physical RAM to accomodate your system load. Personal experice has taught me that it isn't necessary as long as the amount of RAM is sufficient.
Like you, I constantly monitor free space and take an aggressive approach when removing unused items.
Hope this helped, and please excuse any typos, my eyes are very tired.
Thanks,
Nathan