Currently you can't delete revisions from the vault, just whole files. You can archive old revisions - and restore them later if you need them. The archive functionality helps improve performance and will help keep your vault "lean" as you say you want. In 2005 you can delete specific revisions. Beware of deleting things though, as you need to know if there are references that will be affected - a quick where used would tell you that. For now, I would recommend looking at the archive functionality in the admin tool. Just go to PDM/Works help and conduct a search on "archive". That should get you started.
Pete