Unfortunately this is one of those things I can't speak to very well, because I wouldn't consider my marketing efforts for my own apps to be successful. For me, it's been trying to build the best apps I can, and then send them along to friends and other developers I've met and hope they get some use out of it and spread the word. So far, nothing has taken off, but my focus is mostly freelance at this stage anyway so I haven't pushed much. My free to try "Safari to iPhone/iPad" app, Previously, has about 10k downloads, 5k active users, and a substantially lower number of paid upgrades to the full version.