I honestly can't think of much to say to help here and I don't know how good you are with the brush. His armour is intricately detailed in the films, none of which we get on a miniature scale so something else needs to be introduced to break it up a bit. Because it is such a large mini with little colour, it can look quite ordinary even when you give it a good paint job, just because of the lack of tones. Added to this is that it is essentially black metal armour which is very hard to replicate. Most people would do it with dark washes over silver and it seems to work ok.
So my suggestions are:
Black and silver with washes for the most part.
You could try to introduce some dark blue in there by glazing or maybe even a blue & black basecoat. Blue is probably one of the best options, so I would give it a try at least. Do a search for paint stripper, dettol or simple green in the search section as you may want to strip it and start again.
Add a streak of diagonal highlights across the model (I would stop at 1/3rd or half of the way) just to break it up and introduce something different.
Try some NMM or wet blending if you have those skills.
Lastly, I would use tin bitz/warplock bronze on the raised sections as this is good for blending the silver into the black - the blue might do the same job for this, or it may not.
Here is the link for the Sauron painting comp. Everyone did a great job, but Valpas job stand out the most because he used a more selective lighting source and stuck with it, so it doesn't blend like the others. It looks like he did not highlight much in the chest either which makes a large difference.
http://www.one-ring.co.uk/viewtopic.php?f=96&t=22647Good luck.