After having an idea which is the niche, narrow it down to micro niches. For example, if your niche idea is clothing, since clothing is a very common idea then we can narrow it down to "clothing in the polar regions" or "unique clothing in the southeast Asia". Through this, it will be more specific giving you new ideas out of it.
It is always a lot easier to promote your product which you are interested in and know a lot that is why it is very important to know your niche. Also, be sure that your niche would be something that people will love to pay their attention at and would mind them paying for. Be reminded that they will more likely to buy it if your product has no other similar products out in the internet for free. Buying your product would mean waste to their money if they can even have it for free from other similar products.
If you have already a marketing niche in your mind then be sure that you are interested at promoting it and of course you know a lot about the product.
Also it is very important to know if your niche is popular enough to make your business a success. There is actually a tool that is for free, the most popular of them is the Google AdWords Keyword Tool. This tool can make your niche idea to broken down into more specific niches and can also check the popularity of that niches it had just generated.
Last thing to consider in having a good marketing niche idea is to make sure that the people will like and will most likely to buy it. Or if you have a specific target of group of people, then you might consider testing it to them. Through this, you will be able to observe how the people would react to your product.
With all these things set, you are ready to having your internet business a success.