I think it was suggested by someone else here on HN but the idea is to have a widget on blog that tracks how many pages a visitor has browsed. Then simply show him a message saying 'Hey, you seem to be a loyal reader. This is your 3rd visit on the blog and you have read 33 blog posts here. Writing unique content takes time and effort, so please consider leaving a small tip to encourage me to write even more.'

Also show some sort of leader board of donators and the content they consume. Idea is to come up with guilt-ware which presents a new revenue stream for dedicated bloggers :)