I do my own printing with a good, photo quality Epson printer. Unfortunately, all the ink it uses is water soluble. So, when I make prints for someone, I have to make sure they know to keep them dry.
One of the early dilemmas I faced when doing my own printing is that of cropping. The printer software decides on its own of how to crop each photo...rightly or wrongly. I've never used an outside printing service, but I suspect they may do the same. The problem is that the format of your image may or may not exactly translate to 4x6, or 5x7, or 8x10. Nor can one set of image dimensions be used for all 3 sizes without cropping. Fortunately, I found that the cropping capabilities in Photoshop Elements works very well to specific dimensions (proportions, actually). That way, I can choose to what to crop off a bit of the image on the long sides of a 5x7, use a different cropping for 4x6, and so on. Lightroom cropping for printing is more difficult to use, in my opinion, as I've never had any success at it. However, for correcting image tilt and cutting out distracting objects or simply shot 'too wide', Lightroom wins hands down. As I wear my glasses while shooting, 'too wide' is a common problem in my pictures.