I didn't do anything different per se, but I did have a little bit of luck at launch. I generally send emails to some of the top bloggers to try to get reviews, and by virtue of it being good with a really nice looking design, I got 2 good reviews in the same day. Those sales plus any organic sales pushed it pretty high.
I do all the code and I've done varying levels of outsourcing the design. I've done all the design for some smaller apps, I've hired people using elance.com, and more recently I've been partnering with some top level designers (Debt Snowball+ was partnering with a really good designer).
As for spreading too thin, I hope not. I won't be doing all of my apps as I don't have nearly enough time for that. There is only 1 app at this point that makes sense on desktop, and I only plan on doing the most successful apps on the other platforms for the time being. We'll see how it goes, but I think the diversification will be very much worth some amount of spreading thin.
Did you do anything special for Debt Snowball that made it more successful than your other apps, or was it just a good product?
Do you do all the design and code for your apps, or do you outsource graphic design?
Are you worried that developing on 5 platforms will spread yourself too thin?