How can I clean left-over files from upgrades from Tiger to Lion? I've upgraded my system since 2007 (no clean installs). 

Now that I have Lion installed I was wondering if there are any historic files, plists etc. that I can remove as part of a general post-upgrade spring clean?
Are there any other files I can safely remove?
Unlike Windows, the system files for OS X are quite efficient in size and stable in structure, so I don't think you will find much opportunity there to free up disk space.  Preference files are relatively tiny and any system-level bundles were very likely deleted along with the entire folder.  

Where you will probably find the most "junk" is various caches and junk files left over from old applications.  Look in your ~/Library and ~/Library/Preferences folder and remove folders belonging to removed applications.

There is much more you can do, but rather than possibly screwing up your system, I suggest using utility software such as CleanMyMac.  I have freed up 44 GB according to that app.  

Before you do anything else, get GrandPerspective, so you can see what is actually using up space.