Agents and publishers want to know that you have the ability to sell and market your work. If they are going to take a chance on you and your book they want to see it is minimal risk and that you are going to do your fair share. One of the best ways to get the attention of those making decisions is to create demand for your book before it even comes out.
Contact different literary magazines or local newspapers that can run excerpts from your upcoming work as well as share parts online through social media to build up anticipation. As you interact with your beta readers you can get ideas of what parts they found most intriguing Once you know what parts would be best to share you can create teasers and begin to slowly spread them around on different social media platforms and in different groups.
For newspapers and magazines you will need to contact them directly to find out what they would be looking for and how exactly they want you to submit since each publication is different. You don't want to share too much and give away the story but making sure to find enough outlets that it sparks people's interest can be very helpful. Any details you are able to keep track of is also great for when you submit officially to agents or publishers.
Track where you send your teasers and excerpts making sure to note any feedback you receive or how many views your posts have gotten. All of the positive information that you can include (if you can back up the numbers) can be persuasive to an agent that not only are you able to show your marketing genius but also the fact that there is a fan base for your story. Knowing there is a demand makes the pitch easier both for you and for the agent when they approach a publishing house down the road.
Building a demand is not easy but by maintaining a constant press to get the word out there it can certainly give you a boost when telling agents and publishers just how much your book is desired. Being able to back it up is vital so pay attention to what you learn and show your talents for writing and drawing a crowd as you take the next step.