Depending upon your theme you can add css to create a line break or an image separator. Select customize on your theme and add the css or if you can't do that edit the theme's css file (back it up first).

The default guest book uses the class "guestBook-entry". So you might add border-bottom: 1px solid #444; along with a some padding.

I chose to style just the byline by adding a background that matched my theme and a bit of padding.

.contentByLine {
background: rgba(181,189,200,1);
padding: .5em;
}

You could also create an custom template.
hth,
greg