Promised to help and something more fun came up? Keep the promise. People notice which plans you trade away.

Breaking a promise for something better tells people exactly where they rank. The help you agreed to give did not become less necessary when the other invitation arrived; it just became less appealing, and the person waiting on you can tell the difference. Over time you build a visible pattern of whose plans survive and whose get moved. That pattern is more or less your reputation among your friends.

Treat a yes as spent money. Go and do the boring thing you agreed to, and if you genuinely cannot, say so early and offer another specific time. Then make your yes rarer so it means something when you give it.