I am in my third year doing it. First year, I got $2200, second year I got about $4000. This year, I am on track to make significantly less... maybe $2600. Will explain reasons later.

Some calibration: I was already in year 4 of my blog, with maybe 4000 RSS subscribers, when I started the model. Now I am around 6200 RSS, maybe 25-30k uniques/month.

I think the model is workable for most serious blogs (i.e. not random hobby blog or maudlin confessional journal). Just don't try it too early, because past performance will supply the social proof necessary for future performance, and nobody will feel like giving to a blog whose donations in the previous year amounted to $50 from the blogger's own mom.

Another suggestion: use a tipping mechanism like the Buy Me a Beer plugin to get people in the habit of making _small_ donations (under $5) before asking for bigger ones (i.e. "socialize" the donation behavior with a smaller entry drug). I was running the BMaB plugin for 2 years, making a few hundred bucks a year with it, before trying sponsorship. I turned off the beer plugin because it is a book-keeping hassle this year, and the donations have dropped. I think the correlation is causation, so I might put BMaB back in or experiment with other cueing techniques. Since the money is in the nice-to-have extra range and not enough to live on, I don't really care how much I make this way. So long as it covers expenses, I am fine. It's more a feedback mechanism to measure how much people care than serious money.

Other tips:

1. Donations are individual. If you're taking money from corporations, that's advertising/promotional sponsorship with quid pro quo expectations. Be clear about what you are doing. Individuals give goodwill freely and basically expect nothing in return other than that you "keep up the good work" or some such loose expectation
2. There is huge variability in how much you can make, depending on the type of audience and content. I suspect if people see a sort of social mission or clear economically unreasonable amounts of effort in your work, they'll be more likely to contribute. If it is more of a feelings/emotions rather than a thinking blog, that too will increase giving rate I suspect. I fall into the latter category. My posts are typically 4000+ words of thinking and take 6-8 hours. Writing them makes zero economic sense, so it is a labor of love of sorts.
3. Show how you're investing the sponsorship money back in improving the blog. I, for instance, pay my guest bloggers a small honorarium and do occasional events. Basically I earmark all of the money for improving the blog itself.
4. You can get pushy and demanding to the point of unpleasantness if you want to. There are bloggers who set high targets and routinely threaten their audience with the "I'll make this a closed walled guardian with a subscription if you don't donate." Up to you. To me that would ruin the fun of blogging, if I were frequently and pushily asking for money. I do a call once a year, and run a sidebar banner to the sponsorship page. That's it. I might include an article footer as an experiment, now that I've turned off BMaB.
5. Don't be needy/beggy sounding. Well you can, but you give the rest of us a bad rep. Blogging is basically a privileged first-world type self-indulgence. If others are paying you to do it, enjoy, but don't feel entitled to it. You're not a soldier, fireman or emergency room nurse. Don't stink up the blogosphere by pretending like it's a homeless district. If you're really trying to live off your blog income, don't screw around with sponsorships. Do it right: create a membership community, package information products as ebooks etc. etc. If you're looking to make more than 10% of your income needs off sponsorships.

My sponsorship page, if you want to look at a model:

Sponsor

I just use Paypal buttons. Easiest. The levels you choose will matter. Set them based on your guesses about the socio-economic class of your readers. If you're writing a couponing blog for single mothers, think in the $10s, not $100s...