Also, I'd love to make 3k € / month ! . I'll probably be in Vienna for a few months next year, are there Hacker Meetups or similar?
On the website, I get some hits from fileinfo.com. I also posted to a couple of forums where people asked how to open mdb files on a Mac. I tried to be as little spammy as possible, and always disclosed my affiliation with MDB Viewer when writing about it (for example see http://hintsforums.macworld.com/showthread.php?t=27957 at the bottom of the page)
I provide really thorough documentation on my website. I hope that the docs double as search engine landing pages.
I tried Google Adwords (I got 100€ credit for free), but I don't know if that actually led to any sales. I didn't continue because it seemed too much of a hassle.
I don't do much analytics, so I have no clue how successful the different things I did were. I occasionaly check my website using awstats, and I check app sales with AppViz, and that's about it.