Wow, this one is hard.
A very good idea is distinguishing between topics and people (like Kilian did). Maybe some more examples will help.
You usually talk about a topic and generalize it to some point. The German "über" does the same just like in "Übersicht". You ask somebody to tell you about something to get an impression. When I tell you something about a topic I am giving you more like a summary so you get the idea.
What is he talking about? - Worüber spricht er? (Umgangssprachlich auch: Über was spricht er?)
(He is talking about nature (in general)).
When I talk of a certain topic I (maybe) want to convince/win you. I think the contents are more important here.
He is talking of nature and its different animals. - Er redet von der Natur und den verschiednen Tieren.
(Ending the sentence after "nature" in both languages feels odd because you expect something else to follow. At least I do.)
To sum up:
Did he tell you about something? - is generally asking (über).
Did you hear of this thing? - is going further into detail (von).
Some special case might be:
Someone is talking well of you. (von, detail: talking well)
Someone is talking about you (über, in general, no judging)
Speaking of "of" and "about", I'm about to go off (so much confusion :D)