I use more and more remote X11 applications now, and VNC, since I switched my desktop (and laptops) over to use OSX entirely. This works great. Sometimes, it can be a bit annoying to get the clipboards and selections to synchronize across windows, or even to just get a portion of a file from e.g. X11 into OSX. There's a very useful command for this, that can easily be scripted (and bound) to some key of your own preference. For example, to take the clipboard from an X11 app, and put it into the main selection buffer, you can do

$ xclip -o -selection clip | xclip -i

To put a portion of  a file into both the clipboard and the primary selection:

$ head -10 /etc/group | tee >(xclip) >(xclip -sel clip) > /dev/null

You can obviously make some convenience macros around this, or simple shell scripts.