Joe,
Great first try. Some suggestions:
1) Try scribing the mortar lines with a non-sharp rounded tool. I really like manicure tools, which you can find at the dollar store.
2) After you lay down the basic color, try lightly dry-brushing a darker color.
3) To add interest, give some of the stones slightly different colors. I do this using a set of broad-tip markers. Do not make the colors uniform or too solid. Be sloppy (but not across mortar lines). Do it before the dry-brushing (or even before the first painting), which will soften the colors and make them more realistic. A common error is to overdue the color differences which to my eye makes the wall look toy-like (not that there is anything wrong with that if it is the look you are going for).