Canon 5D III & Cokin Z Pro Graduated Neutral Density Filter
Circular polarizer
A Circular Polarizer is another piece of optics that must be in your kit bag. No software can mimic the effect of a polarizer. It helps to reduce reflexion of light & increase contrast. They are perfect for changing the tonal balance of the sky, reducing glare and basically cutting through the haze of diffused light that can spoil a landscape shot. Hoya, Singh-Ray and B+W (not Black+White) are some of the companies that make both Grad ND Filters and Circular Polarizers. Read more details on polarizes here
Get those the long exposures right
Mastering the art and science of long exposure is critical for you to be a successful landscape photographer. Latest cameras can handle exposures quite efficiently, it need not be too complicated though. For complicated situations, there are some tried, tested and proven methods of getting the long exposures correct. Mastering the basic three fundamentals of exposure triangle covering ISO, aperture and shutter speed is critical in this case. But with that you need to also be able to control the amount of light that enters the camera; both through the lens and through the viewfinder. Yes! The viewfinder can also let in some ambient light in and to control that you can either use a black cloth or can use a viewfinder cover that comes with some cameras.