The purpose of your product preview is to give the buyer a good idea about what is in the product. However, if you give away too much, there is no reason for the buyer to actually purchase the product. It is a good idea to include your cover page and teacher information page/table of contents in your preview. Information about your products and a link to your store is also a good inclusion (at the end). As for the rest of the preview, there are several different approaches.
Most sellers include sample pages in their previews. Many sellers intentionally make their sample pages unusable by placing a large "PREVIEW" or other word across the page. You can see that this is what Laura Candler did here. One advantage of this approach is that you can include more pages since they are not usable. This is also a good approach if your product only has a few pages. Of course, you must save your preview in a PDF to make this work, but PDFs are actually preferred by many buyers.
Many other sellers (me included) want buyers to be able to try the product, so we do not make them unusable. My own thoughts are that if they try it, they may come back to buy. I realize, of course, that many teachers will just use the free pages and never come back to buy but I like giving away a little bit for free.
Some sellers create thumbnails of either all or some of the pages in the product. This can be a really effective way to display your content without giving away usable pages. Take a look at how Caro Carroll did this here. The easiest way to make thumbnails like these is to use your snipping tool (find it in Accessories). I made this thumbnail preview (before I knew about the snipping tool) by zooming out on my product so that all the pages were visible on the screen and then taking a screen shot and editing it Paint. Thumbnails can be very effective, but if you idea is really original and fairly easy to copy, you may not want to have the entire product out for everyone to see (and copy). And yes, copying another person's product is illegal and unethical, but unfortunately, people still do it.
Part of the Page
Some sellers include only part of a page in their preview. Here is an example from Teacher Tricks. I personally find this approach to be confusing. In many cases I don't know what the purpose of the page is since part of it is missing.
The Combination Approach
Some sellers combine samples with thumbnails, which can be extremely effective.
Make sure that your preview is formatted properly - I have seen many previews with skewed margins, page breaks in the wrong places, or a blank page in the middle or at the end for no apparent reason. One way to think about previews is to imagine that you are the buyer. If you downloaded the preview, would you understand what the product was about? And further, would you buy it?