Just ask for it. I remember putting a "Buy Raganwald a Coffee" Paypal link on my blog and presto, people started clicking the link and sending me $1.27 each. I tried raising prices: I changed the link to "Buy Raganwald a Double Espresso" and people started sending me $3.15 each, and numbers did not fall by 2/3.

Start by asking for donations. You may be pleasantly surprised by what your readers will do for you.