what has worked best for me is to create a folder on the host computer, call it newstuff, playlist, or whatever, but keep the size below the player's limit (8gb for example). then sync this folder to the rockbox folder with a robocopy script. So, then you'd just add/remove stuff to the host folder as needed, connect the device, then run the sync script which adds/removes files to rockbox to match the pc folder.
there are probably tons of mirroring programs that would work similarly if you don't want to use a script.