This is great advice Todd. I personally drew up a blue print of my novel before I started it. I jotted down character outlines- their physical attributes, personality traits,their main role in the book and the specific chapter in which I intend to introduce them. I drew lines connecting these characters and noted the chapters in which they meet and their interactive nature in the same.
This helped me live as those characters when I wrote about them.
And yes, I am just in the process of finishing up my first ever historic fiction. Please throw more light on the subject of agents. It would be of great help.