How about giving up magical thinking? I don't know if it would be possible though as it seems to creep in at the margins.
The Rationalist Pope recommends giving up things with the property that it is easy to tell if you've accidentally stopped giving them up.
As I understand it, Lent is a holiday where we celebrate the scientific method by changing exactly one variable in our lives for 40 days. This seems like a convenient Schelling point for rationalists to adopt, so:
What variable are you going to change for the next 40 days?
(I am really annoyed I didn't think of this yesterday.)