Ask yourself:
* What do I know that gives me an advantage over the average joe?
* What other resources do I have that give me an advantage over the average joe?
Generally speaking, if your idea to make money sounds really lame, boring, risky, complicated and/or esoteric to an uneducated layperson - but due to your domain knowledge you know that it's not - you're on the right track. Bonus points if it benefits from obscure, difficult-to-get resources that you already have (for instance, a best friend who owns a niche website you can advertise on, or family connections in the X industry, or you live in a town that has a huge Y industry.)