I save as .tif files & for some reason have better results saving in RGB format even though I'm printing on a CMYK printer (haven't figured that one out yet)
also, it probably goes without saying, but high enough resolution for the size you will be printing is also important. I go with a minimum of 100 dpi AT FULL SIZE. On smaller prints, I go up to 150 dpi or more if I am really looking for photo quality, but typically on a large format printer, we are printing stuff to be viewed from a distance, so anything over 100 dpi wastes time for no good reason.