1akal,
The answer to this is one that I have thought about very carefully in the past as we have thousands of pages on our website.
I first of all did this programmatically taking the first 150 letters of the body of each page, or description of a product.
I soon realised that the search results weren't exactly what I was after and soon found myself manually updating the extracted text to suit the product and search keywords.
Over the years I have edited the majority of the pages manually and constantly update the information as keywords drop out of top ranking pages.
You get out only what you put in in this case and if it means manually writing the content then so be it I'm afraid, but you may want to take the initial step of extracting some of the rich content in there first as a starter.