This point was raised a while ago by Kevin Riley, but it is something very important and worth highlighting. It is something I have caught myself doing in the past and it can be a success killer.
Have you ever come across a technique, or a business model, a niche....and then thought "Yeah, looks interesting, I'll give it a go". Then there comes a point with that campaign where you realise to get it going, you would need to either spend money on PPC advertising, or SEO backlinkng, or a press release or whatever, but you don't because you want to see if it will make any money first before you invest in it? If so, you are 'taking a stab at it'. If you went in with the mindset of "knowing it is going to work", then you wouldn't think twice about handing over the money for what is needed.
Looking at the bigger picture, people that DO make a lot of money normally have something in common, they do something until it DOES make money. The problem with online ventures is it is so cheap to get started, so it is easy to drop the project and move on.
So, if you can see that the niche has real potential, or even better, has worked out well for others, you must either go in full steam, no holds barred, or not at all, as it probably won't take off.