Here's what I do. We just started practicing this week. During the first week, I keep track of attendence; but, it doesn't really count against them if they miss. When they do miss, I let them know if they had an acceptable excuse or not. If it's something unacceptable that they plan on also having to miss practice for in the future (work, rec sports practice), I tell them they have the rest of the week to straighten it out to where it doesn't interfere with cross country practice. If they can't straighten it out, they either have to quit cross country or quit the other activity. Starting the second week, every missed practice counts against them. I usually give them 2-3 unexcused absences for the rest of the season before giving them the boot.
I do work out different practice schedules with my runners who are on the school swim team, participate in band and other after school activities, and leave school early to go take college courses.
I usually don't make any compromises for non school related activities.