If you are computer savvy you might already have an image editor. Most images can do with a bit of a lift in the pre-digital days photolabs would enhance the image duriong the printing process by dodging and burning-in. Even now you will find the latest printing equipment makes changes to the image - enhancing shadow detail for example. These are all quite simple tricks. Once the basics have ben mastered then making the next step is logical and easy to do.
Most digital cameras offer a choice of image format ie. jpeg, tiff or raw. The most widely used format though is jpeg, mainly because it produces a smaller file without too much degradation. The downside of using jpeg is that it is all too easy to compress the file again making it even smaller and degraded to a noticeable degree. The first thing to do, when image edit a jpeg, is to re-save the image in a different format. For example saving a jpeg as a tif file will prevent this from happening and if you then look at the properties of this new file you will notice how much larger it is. As an example an photo taken with a Canon 350d might be 3,643kb but when opened in photoshop and fully decompressed it will be 22.8M. Now if this image is re-saved as tif or photoshop psd file - without compression - it will be 23.36M. However, if you save the picture again - even without doing anything else to it - it will resize down to 1,904kb. Your image is degraded, next time it will be worse and so on. If you are worried about filling up your harddrive you can delete your tif (psd) file once you have made all the changes you need.
This first step has to be the first thing you do - that is (again) before you edit your picture save it as a tif or psd file. Edit it save the tif and keep it until you are sure its ok. If you need prints from a supplier that will only accept jpegs thats fine just re-save your file as a jpeg at maximum quality. It will be as good as the original, better, hopefully, if your editing was ok. For editing I do mean anything such as cropping, red-eye correction, swapping body parts and etc.