If you use iTerm or Terminal, you may be used to press cmd-k to clear the current window. If you're inside a Tmux session, this is a bad idea.
You can however use the following snippet to achieve the same result inside the current Tmux pane:
bind -n C-k send-keys -R \; clear-history
Just press ctrl-k and you're done! Works really good when you're tailing logs.