I always have my final artwork done and have it right there when I work. I also figure out each step ahead of time. I have used edible markers to sketch onto the cookie. Many times I will copy my design onto wax paper with edible ink, lay that over the top of my flat, dry RI'ed surface and use a fine point turkey skewer to trace it to create a faint line that I can follow. If any marks show after you are done piping, use a paintbrush with a minimal amount of water to make them disappear. It will eventually rip up the wax paper, but then I just make another sheet. But these are for the more intense designs. I am trying to work towards some simpler ones so I can freehand them, but still have all of the steps figured out ahead of time.
Like Jen, I also like to do some images in RI separately and then drop them on on the image is dry. It allows me to be more precise as I put my image underneath and then trace over exactly it with the RI. I will use foodsafe acetate, wax paper or parchment paper when I do this. As Jen mentioned, I always practice my lettering separately first by putting the words under wax paper before attempting to put it on a cookie freehand. Lettering does not come naturally to me.
Eventually I want to try stenciling as I just love what Cookiemookie does! It is so perfect, just lovely and looks like a real time saver (something I need to work on!
Attached is an example of both the the wax paper idea when I needed to get lettering precise on some Chamber of Commerce cookies along with an example of RI-ing the image separately in order to drop it on an then the final cookie. These are two of my favorite techniques.