[tetris11]

C-b [ goes into copy mode,then you can traverse the screen with arrows.

C-Space marks a region, move arrows to select text, and M-w to copy to clipboard.

q to escape copy mode

[stereolambda]

I'm used to selecting only with Space, and confirming with Enter. C-b ] pastes.

This seems to be to the tmux's own clipboard. When you want to copy externally many lines, you can use C-b Z to zoom or use a lame workaround with text files opened in gedit or something. (It's a marginal use case in my experience, so I didn't bother to optimize.)