Great product... goes without saying doesn't it - which is why I won't talk about this much. It has to be compelling, plenty of scope to scale and return significant value. It should be defensible like Genadinik said, i.e. hard to copy.
Compelling, passionate founder(s)... is really important. Often a startup will get investment purely on the back of believing in the person, rather than the product. I know angels that have backed people knowing that even if the current venture doesn't work, a future one will because they have a high degree of faith in the founder.
Paint a picture... as you already have a product that is out there you'll need to show where it can go from here. Plan. Focus on painting the vision of what it could be, given the appropriate investment and backing. Use the product as it is today as a starting point and evolve it out 2 years, 5 years even. Demonstrate clearly how it will result in significant value to the investor.
Some angels like being hands-on... If you find an investor that has key experience in your area and is likely to want to help if they invest, tailor your pitch to them and show how their involvement is likely to accelerate the growth and success of the company. People like to feel wanted and useful.
Hope that helps.