I'm a chronic under-estimator. So, whenever we're in grooming or sprint planning, I make it a habit to ask the other members of the team if the size/hours make sense, or are straight out of "Al's world".

In those occasions where I'm asked to provide a WAG to an executive or salesperson, I always multiply my first thought by 150-200%. Maybe even 300% if there's an external stakeholder (3rd party integration, something for a specific client, etc).