In Photoshop, when you use the Crop Tool, you can (at the top) actually specify (and save your most used) the size you want it to be printed as. Type in the inches/centimeters wide and high, and at what dpi you want, and the Crop Tool will be constrained to ONLY that size until you hit the Clear button.

I just shoot normally, edit normally, and save as a TIF when I'm finally finished - this is my "master" image. Whenever I want to print it or HAVE it printed (photo lab, etc.), I use the Crop Tool in the above fashion, and then re-save the cropped image with a new name denoting the size. Example:

ErikBDay0027_46.tif - the very end denotes a 4x6. When it ends in 57, or 810, guess what it is...? It becomes much easier to not only PRINT the right size, but I can keep track of what's been cropped, and the final crop is repeatable.

Hope this makes sense and helps-
-Lew