I think the best decision I made was finding a niche (UI development for iOS) and sticking with it, trying to build up my knowledge so that I was a very knowledgeable person in the field.
The niche worked out because it's what I'm most excited about :)
I use Quixly for payment processing, it's the service that Pictos icon set uses because Drew, the creator of Pictos, wrote it himself. It's outstanding and very cheap. I highly recommend it for selling digital goods online.
