How to disable screensaver and DPMS while mplayer or skype work? I don't have a screen saver, I just have assigned a key to slock. So I don't know how to disable your screen saver, that's likely configurable with screen saver settings.
However, depending on your screen saver, it may listen to xset (or not), in which case you can do: $ xset s off -dpms to disable both it and DPMS. You can even use this in a function in .bashrc called mplayer and then turn it back on again to have this toggled whenever you start/quit mplayer. I personally have it in a script for when I want HDMI output. BTW, next time, please remember to check the Arch Linux Wiki first, it has lots of useful info (even when it's for another distribution/ distribution-agnostic problems): https://wiki.archlinux.org/index.php/Display_Power_Management_Signaling
Have a nice day! To you too. João Miguel