The easiest way is to calculate the camera position based on the focus point (a point on your ground, or whatever).
//Lets start with looking at origo for now.
Vector3 FocusPoint = Vector3.Zero;
//This tells us where the camera should be, RELATIVE to the point we are watching.
//I set this a little up and a little back
Vector3 CameraOffset = new Vector3(0f, 20f, 20f);
Matrix ViewMatrix
{
get
{
//The Offset is just up and back, we need to rotate it 45*
var rotatedOffset = Vector3.Transform(CameraOffset, Matrix.CreateRotationY(MathHelper.PiOver2 * 0.5f));
//Now we can create out viewmatrix. No need to use a transformed "up" unless it's not going to be upside down or something.
return Matrix.CreateLookAt(rotatedOffset, FocusPoint, Vector3.Up);
}
}