Bawylie
A very OK person
When I ran Sunless Citadel recently (TftYP), I set it up where you had a character and a backup character. You earned Inspiration as I described in my "Case for Inspiration" thread, but when you spend it, it had to be on someone else's failed roll and this added +1d6 to it. At the same time, your backup character earned a set amount of XP. The idea here was that the backup characters were in the Yawning Portal telling the story of the "active" characters' adventure and spending Inspiration was someone's backup PC telling other backup PCs, "That's not how I remember it..."
This was wildly successful in getting the players to earn and spend all their Inspiration in a given session. Guaranteed 20 Inspiration earned and spent in a 3-hour session across 5 players. As soon as someone failed a roll, the players were racing to be the one to say "That's not how I remember it!" It was hilarious and fun and played up a group of background characters arguing over the details of a story. It arguably encouraged the players to pay more attention to what other people are doing as well. I probably won't do this outside of running TftYP adventures, but it sure was fun.
This is the method I’ve been using lately and it’s crazy-pants good.
You use it to improve someone else’s roll but you get XP for it. Up to 4 times per session. Players are scrambling to use it.