Graduated neutral density filters, how I love thee, let me count the ways.
I bought the best money could buy (spent about fifteen hundred bucks on a few bits of glass: 2- and 3-stop grads in both hard- and soft-step varieties, as well as variable neutral density systems with integrated warming polarisers that work from 2-8 stops). This was taken with the variable system at about 4 stops, from memory, and a hand-held 2-stop soft step:
http://users.tpg.com.au/octane2/md.html
You can't go wrong with Lee filters, though. For some reason, despite the worldwide shortage, they're readily available on eBay -- brand new, from legitimate suppliers. Don't buy Cokin's. They're not neutral density, rather, grey filters, and, will leave horrible purple and brown casts that cannot be white balanced out.
You need a good quality circular polariser. Hoya Pro1D series are very good, and, have a slight warming effect which helps with natural in-camera saturation.
As mentioned, I don't use filter holders, preferring to hand hold. Yeah, they get scratched this way, but, the scratches have never shown up in any of my work (and, I shoot at f/16 most of the time, which, I know is well past where diffraction effects are theoretically supposed to turn your image to poo (around f/11 on the 5D Mark II), but, it just doesn't). The advantage of handholding filters, particularly when using strong densities, is that it can help prevent gradations showing up in your image. By carefully (slowly) dithering the filter in a random or circular motion for the duration of the exposure, you will go a long way to making the use of a filter subtle.
Hope this helps, some.
H