I feel I'm constantly "contributing" to the support of the Cal State and UC schools as a taxpayer. I get those calls from the Foundation, and so do my parents, almost 20 years after graduating.
I do try to hire grads from my alma mater, and I do offer internships (paid and unpaid) to students at my business.
I do give money to the Gamma Phi Beta Foundation, and I feel pretty good about it, because I know exactly how they use the money, and they operate in such a way that very little of my donation gets eaten up by admin/fundraising expenses.