Alright then, when does 'prone' go away?
Well, Grymor beat me to this explanation (and did a great job) - I'm just adding more detail...
That depends on the strength of the aspect. I'll see if I can run through an example (This may just be how our group handles it, but I'm pretty sure it's within the RAW):
Character X wants to knock character Z prone. This is a contested maneuver - X would use an appropriate skill (Athetics or Might, for example), and Z would get to defend using similar skills. X has to beat Z to place the aspect "Prone," but how long it lasts depends on the difference in the roll. A tie places the aspect for 1 exchange, but it's "fragile" - 1 tag and it's gone. Any shifts in excess of that makes the aspect "sticky" for that many exchanges. (So if X beats Z by 2, the aspect is sticky and stays for up to 2 exchanges - taggable for free, plus further FP invokes).
Duration depends on Spin.
For Z to get rid of the aspect before it runs out, Z has to do a maneuver of his own - essentially a "counter-maneuver." In this case, I'd argue that Athletics is really the only viable skill to remove "Prone" (But hey, get creative!) If no one is around, hit a 3 and you're done. Of course, if X is still there, it becomes contested (X would want Z to stay prone), with Might or Athletics to "defend." As long as Z beats X, the aspect goes away.
You may want to "counter-maneuver" with a fragile aspect if you have a chance, but it's much more relevant with sticky aspects.
I showed a variation on this in a webisode on the Dresden Files: Dallas site where "Disarmed" was the aspect in play:
http://www.obsidianportal.com/campaign/dfd/adventure-log/wardingOn the note of block vs. aspect: if you want to *hold* the person prone, that sounds like a grapple to me, but that just my opinion.