I do not regularly donate to my alma mater because I am just as poor of a student as I was when I left.
Okay, I'm in a better situation but still not enough where I have a ton left over. I have donated to the scholarship fund a couple of times but not yet regularly. When I am in a better financial state, I full intend to be more financially supportive. However, I keep in contact with my old psychology department and, when I'm in town, I always stop by, speak to classes about what grad school is like, and give out my contact info in case the students have questions about what life is like on the other side. I have also supported productions from the English department since I was an English minor. I am more than happy to give back in that way because of how much my undergrad gave to me.
My previous graduate institution is a whole different story. I wouldn't give them a quarter if they needed to make a phone call. Unless I need a transcript, I usually deny I ever went there.