Imagine you have a paying customer who really loves your product and who is riding the elevator with a friend. What would your customer tell his friend during this brief ride to convince him to try out your product? Write down the stuff and use it as a foundation for your copy writing.
Pretend you are a consumer who is doing a comparison between you and your competitors. Write down the qualities that all the products share and qualities that your competitors boost about in a comparison table. Use this as an the groundwork for writing your landing page copy.
Take a look at what your competitors are saying. Chances are that they have already done a lot of the research work for you, this especially true if they have deeper pockets than you.
Use Google adwords to figure out which words or phrases interest people the most.
Start a/b testing right now. Google's Website Optimizer is free, Visual Website Optimizer is a praised alternative.
Demonstrate it, showcase it. If you suspect that your customers might have trouble understanding your product, clearly demonstrate its basic usage in a video that you display on the landing page. If possible, showcase how other customers are using your product.