I've never been really happy with any of the metal aircraft I've skinned. If you give it a "chrome" type finish it looks great on the bmp but a bit bogus in the game except from certain angles. Unless reflections move as your viewing angle changes, they look like they've been painted on.
So I keep the big reflections effects (wing on fuselage etc) fairly faint. Instead concentrating on smaller stuff like rivet hollow highlights and the difference in angle between two adjacent panels. The upper surfaces get to be a bit bluer and the lower ones a bit greener.
The opposite problem is stopping it looking like grey paint. One thing I do is to make two duplicate layers of panel line highlights. I put one layer below the paintjob at high opacity and one above at low opacity. This makes the metal appear shinier than the paint.
I think the standard HTC metal effect is too reflective, it makes the skin look washed out. I use a material file with the specularity reduced to 0.5 or so, about half way between full matt and full gloss. Then I weather the hell out of it, if its dirty it wouldn't be that shiny anyway.
I think you could vary the individual panel colours a bit more Citabria, look at the cowling panels on the photo.