This may be trivial for some or many, but as I stumbled upon it just recently, I though I share it here: Making your headset work with Skype on the Mac (and maybe also on Windows) requires that you choose your input and output device in the Skype preferences.
Sounds as no surprise, right? But my first attempt was to just set-up the headset in Mac OS X System preferences, which was not sufficient. While I heard some sound and in my Logitech headset I could even hear my own voice, the other person at the other Skype end could not hear me.
So, you don’t need to go to system preferences, you have to set input and output devices in Skype > Preferences:
Happy phone calls!:-)