Your environs set the tone of the daily tasks.
I've done sys admin in different departments of one corporation so that my duties were altered depending on user needs and/or system setup.
i found it most effective in figuring out my daily/weekly/monthly tasks by creating my own UNIX sys admin manual. Each time a new task came up (due to user needs or system requirements) i added it to the manual.
But i was constantly referring to sys admin books. Here's a site that houses the Sys Admin Guild's bookshelf:
http://sageweb.sage.org/resources/bookshelf/general.htm
Keeping an accurate log of my daily tasks has been one of the best methods for helping me acquire a working knowledge base for future tasks.
Hope that helps!