This video (slightly out of date as skype didn not go away) shows how to adjust your callers volume.
If you're on a Mac check out Call Recorder from ecamm http://www.schoolofpodcasting.com/callrecorder
This one does similar but shows how to use a portable recorder (I usually use both)
Here is a great video from my friend Ray Ortega about how to setup a mix minus.
Please let me know if you need more help.