Hello, thanks for playing!
Depending on how often I want crits to happen, I use on of these two rulings:
- Critical on double 1 / double 20. These must be spectacular (as they have 0.25% chance to happen)
- Critical on Fail with at least one natural 1 and on Strong hits with at least one natural 20
First ruling makes critical rolls particularly rare, and particularly important, whereas second ruling makes them more likely to happen. Of course, even with the second ruling I tend to give double 1s / 20s special treatment ;)