For our alternates we tell them right up front when we post our team: Team members: blah blah blah, Alternates: blah blah blah.
Alternates are required to be at all practices and know ALL parts of the routine respective to their stunting position. This means knowing all parts of the dance, etc. so they can rotate in with anyone at anytime. They pay the exact same fees as regular team members. We tell our alternates they have the hardest job since they need to know the most counts-- usually at the end of the season we give them special recognition at the banquet for their time and dedication.
We have not had an alternate two years in a row, however, I wouldn't say it could never happen.
So at choreography we have the alternates "Shadow" someone in their stunting position so they also can mark the counts and rotate in. Then if someone is not at practice, they immediately step in so we can continue to do full outs.
If someone throws a "fit" about being an alternate, then they aren't on competition team. In our opinion, if they aren't willing to take the time to progress on the mat then they don't have a good enough attitude to be a part of the team :) No one "gets" a spot on the mat- they all have to earn it. For members who aren't alternates, they know that at any given time we could switch them to an alternate and have an alternate take their spot on the mat for a variety of reasons (stunt not hitting, attitude, attendance, injury, grades, etc.)