I will admit to knowing b*ggerall about space marines, but it\'s looking good so far, especially for only your 3rd go.
Suggestion #1: I would suggest a little dark-lining between the gold and the blue, to help define it and make it pop out a bit more. In this case a very dark blue might be better than black, which can look very harsh in the wrong place.
Suggestion #2: Wash over the blue armour to even out the layers a bit. Choose a darkish mid-tone colour, and thin it down really well, probably about 10 water to 1 paint. Then paint it lightly over where you want it- try not to let it pool too much. You will probably need a couple of goes to smooth everything out. Then pop on another highlight and a lil shading if required.
Suggestion #3: You can buy paint mediums, certainly, but as an effective interim measure dish-washing liquid works very well. Just put a tiny drop in your paint water. It breaks the surface tension and helps stop that annoying ring effect.
I look forward to seeing the next stage!
(Edit: I can\'t spell, apparently...)