Relevant - Notifications are generally interruptions so your message should be something your users need/want to know.
Timely - This ties to relevance. If your message is stale, it's not going to be relevant.
Keep it short - You're limited to 140 characters to help you focus your messaging. Choose your words carefully.
Link to Challenge when it's Challenge related (e.g., "Big Lead Change! Team B just overtook Team A!")
Review in both Previewers - This should help you ensure that your message is coming through as desired.
Don't overdo it - Every user has the choice to turn off Notifications from the app and they'll exercise that choice if the app gets too noisy.