QUOTE
The biggest mistake I make is constantly overwriting render files. If you don't change the file save settings in the render panel it doesn't warn you that you are rendering over top of a rendered file from another project. Luckily everything I render is backed up in like two or three different places after it's rendered... but it's still a bit confusing... I once couldn't find a render because I had no idea what it was called or where it was. I had to search on the date/time before I figured it out.
I plan to look again at Glenn Anthofer's 'Render Done' utility as for awhile I used it to continually render to one place.
Once the render was done it'd launch a batchfile that would copy the folder into another place (the method I liked best was to name the folder as the date... format: YYYYMMMDDHHMMSS. A bit too much of a hack I admit and it doesn't work on a MAC. Now that the 'Apply' button makes rendering more sticky I really need to try RenderDone again.
Edit: The danger of course is that if something fails... you may have overwritten a whole lot o' files!!!
I've been rendering again to one place a lot lately.
I have a folder inside a directory named RenderFolder. After a render, I copy and paste the folder into the same place via a quick Ctrl C/Ctrl V. If the folders name was 'Output' then Windows names it 'Copy of Output' automatically. Subsequent copies produce 'Copy of Output(1)... 'Copy of Output(2)... and so forth and so on. Later I switch hats and go into Editor Mode where I rename and organize whats been rendered.
Why go through all this trouble? I really don't like rendering.
I like seeing them perty images though.

With standardization on PNG images I've been developing an external workflow that helps me better see the images as they are rendered... right inside A:M. Some of it is even... dare I say it...
Change your Pants friendly! 
Edit: So my point here (almost forgot) is that I don't care if I overwrite files. My workflow automatically overwrites them unless I decide otherwise.
Obviously this works best with simple/low resolution renders. It'd take entirely too long to copy huge renders to other locations on the fly.