1. If you ever go to places with nearly no cell signal, consider turning on Airplane Mode temporarily. Weak signals burn battery.
2. Log out of Skype!
I don't know how Apple lets Microsoft get away with Skype's crimes. Every other communications app can let you be logged in to receive chat requests without burning any CPU, RAM, or battery until the moment you are actually ON a call. But Skype needs all of those resources just to be logged in.
3. Two-way video (FaceTime too, not just Skype) burns battery while in use. Save the long calls for then you're near power. There isn't much to be done about that: the device is encoding AND decoding, sending AND receiving lots of data, non-stop, while the screen and speaker remain on.
4. Power fully off and back on; just in case some "bad app" has gone nuts on you. Eventually the system will kill it, but maybe not right away.