The key is to capture the largest size and best quality image possible with no camera movement, correct exposure and good focus in the original photograph, shoot raw if you can (DNG) or the lowest compression jpg.
Then use good software to tweak the photograph in post. Adobe Lightroom is the best S/W, a good investment if you like taking still photography and printing. Make sure the final image has good contrast an is correctly exposed using the histogram (use levels in the final tweak to make sure the highlights, shows and mid tones are all good)