To write great copy,
it helps to be able to visualize your audience. Sales copy should read as if you are actually chatting to your customers, and if you can imagine you are talking to someone that fits the demographic you are aiming at, your copy will be more convincing. How old is your target audience? Are they male or female? What is their average income level and what is their level of intellect likely to be?
To recap, the things to bear in mind when writing killer product descriptions are your structure, style, length, and grammar. Make sure you are selling directly to your target audience, and that you include some additional elements that will make your customers feel more comfortable about buying from you.