The goal, is the for the student to keep all of their coins across the session. In this session, my student was reading cards while I listened for good /r/ sounds. If the student says a word with an /r/ produced incorrectly and didn't self-correct, I took a coin.
Of course you could use this same idea to work on other skills. I've used it for pronoun carryover this month, so if a student makes a error in conversational speech with subjective and possessive pronouns, I took a coin. I also usually start with around 5 or 6 coins with students just learning to work on carryover. You don't want the student to run out of coins in the first 5 minutes of speech!
At the end of our session, if the student had any coins left, they earned an extra star on their chart. If I got all their coins (3 for me!) then they had an extra sheet of homework go home.
This has been super motivating for my students and helps us work on carryover. How have you worked on carryover with your students?