Another tip I've learned from the local printshop is to sometimes apply a light blur on images that become grainly when you increase the image size. Use with caution though and do crop print samples (you increase the image size and then just crop a say 10x10 inch block of it to print a sample) with different variations in the effects. Remember that high quality sharply focussed photographs are the easiest to make big posters and bill boards from for obvious reasons!