I haven't personally looked at how Snapzu does it, but I did donate once a year after about 3 years of lurking. I figured that the amount of time I spent reading forums made them deserve something from me. Now , however, I'm not sure how I feel.
Snapzu does it through paypal, which makes it slightly more annoying to setup monthly donations, and makes me worry about their account getting locked. (Paypal has some ridiculous flags, and they are difficult to work with)
I'm in a similar situation myself. Donated once or twice a year while lurking most of the time. I'll have to take a look at how its done considering the amount of time I'll most likely spend here.