In case you are new to linux and have been using windows by now, you may notice that copy and paste function does not always work after the application which holds the copied content has been closed. Example of this is iceweasel. If you copy some text in iceweasel, and then close the browser you may notice that the copied text is lost and is not available for pasting anymore.
Solution to this problem is an application called clipboard-daemon, available from:

http://members.chello.nl/~h.lai/gnome-clipboard-daemon/clipboard-daemon-1.0.bin.tar.bz2

Extract it, copy it to /usr/bin/
and then add this line in your ~/.xsessionrc : clipboard-daemon &
If you don't have .xsessionrc file in your home folder just create it. Remember the dot before .xsessionrc
The daemon will start automatically and protect the content of your clipboard