Keep your lens clean. This might seem obvious, but it is often overlooked.
Your images will lose resolution and contrast if you do not keep your lens or protective filter clean.
Use a lens or spectacle cleaning cloth, but you must make sure that there are no small particles of grit on your lens before you clean it by examining and gently blowing on it to remove any foreign bodies.
These can scratch your lens. Once you have determined that your lens or filter is clear of small particles, breath on it to create some condensation, and gently rub your cleaning cloth in circular motions.
I normally do this procedure before every shoot.
If you have any questions about any aspects of photography or camera choice, please email firstname.lastname@example.org and he will try to answer your questions.