I have a google doc called "The No Project".
Every day I try and add something that I have said "no" to. Projects, feature requests etc. I don't always have an entry, but it keeps No top of mind.
Not exactly a constraint, but....
The problem with that is you become the "no guy" even when yes is the right answer. Saying no to bad things is good, but you don't want to look too hard for reasons to say no lest you say no to something worth doing.