As a middle-aged alum with a higher than most income, I'll share my approach:
1. I don't donate to the school. Even though I received a partial athletic scholarship, I feel I gave NU more than ample money in tuition for undergrad and grad. Before you suggest my education led to my income, I would point out that my intelligence afforded me access to many top tier schools (NU was closest to home and Big 10) and I feel they would have all prepared me equally. Therefore, I do not feel NU did anything particularly special to cause my financial success. My career path is not a result of any alumni, coop or intern connection. Also, with $7-$8 billion in endowment, I think NU is doing just fine on its own.
2. I have purchased many athletic tickets and once held season tickets for each basketball and football. I have traveled to Bowl games and gone to B1G bball championships regularly. I also contributed in a small fashion to a fundraising drive that followed one of our successful football seasons - they just caught me at the right time.
I would probably donate to athletics again - but not today. Today, I pass on buying, or receiving, football tickets (I couldn't pass on the tickets to ND last season and glad I didn't...). I would buy basketball tickets and plan to do so. If I am engaged by the team, then I don't mind spending money on the program. (I actually went to a few softball and lax games in recent years.)
But I do not feel that the administration and the head coach are placing program success in football above their egos. I feel that Fitz is arrogant in his loyalty in a stick it in your face fashion. I feel the admin turns a blind eye because Fitz is such the feel good story and a good face for the program. And I think this is not what I want to support in any meaning way.
As an aside, I wonder if NU used some of that endowment to offer some form business incubator for its alum, perhaps they could not only help those that left NU with giant debt get started, but also encourage, or even contract, for future contributions from a larger contingent.