Is it possible to hook automator to the "before download" event in Safari?
You can hold down the control key or click with your right mouse button and select save as... 
But I must admit that doesn't work always because some downloads aren't clicked on.
I personally dislike this solution because you have to click more ( but save as already gives you more to click then needed in my opinion ;) )

You can do this with this workaround using automator:


  
  Open the Automator tool
  Create a new "Folder Action" type document.
  Assign the Folder Action to the Downloads folder.
  Add these actions (in order): Move Finder Items, Reveal Finder Items,
  Name Single Item.
  Set the "Move Finder Items" action to use the Downloads folder as it's
  default (or whichever folder you are most likely to move things to - I
  use Downloads because most of the time I'm moving files into an
  archive structure and want it to be easy to skip this process by just
  hitting Return and leaving the file in Downloads when I don't do
  that.)
  Open the options for the "Move Finder Items" action and tick the
  checkbox for "Show this action when the workflow runs."
  Open the options for the "Name Single Item" action and tick the
  checkbox for "Show this action when the workflow runs."
  


As quoted from: https://superuser.com/questions/201298/to-prompt-a-folder-location-when-safari-downloads-a-file