I've donated to another all pet forum, I don't mind throwing a few bucks towards keeping a site I enjoy running. It included perks like a "members only" subforum, increased PM storage, etc but that's not my real reason for donating. The way it was set up, any donation made (if you could only afford $1, that's cool) would put you in the donators class.
I think even a button just to be able to donate a few bucks here and there through Paypal or something would be nice.
But, with all this, there does come the headache of managing the premium members. Such as people with premium memberships getting banned (and moaning because they paid...), people wanting their money back, etc.