Gun barrels

By far the best way of achieving a realistic look on gun barrels is to paint them black and "metal" them with powdered graphite with a stiff brush (or something similar).

Exhausts

I'm talking about ejector-type exhaust stacks on piston engines here--I always paint them in a Humbrol enamel colour entitled "rust". When dry, I dust on some black pastel and rub it in with the brush, which gives a nice brown/black stained look.

Tyres

I used to paint them matt black, but now usually paint them in a dark grey (something like Humbrol 27), which gives much more scope for oil washing/drybrushing to bring out the detail and allow weathering.

Aerials

Another popular bone of contention!!! I am not a fan of stretching sprue, which I think gives very variable results, and I’ve better things to do with my time than messing about with sprue and candles on a winters eve! I have used 1lb breaking strain monofilament fishing line, which doesn’t stretch over time, looks good, but is an absolute sod to glue in place! Superglue and patience are the only ways to ensure consistent results. I’ve recently got some extremely thin plastic rod, but this looks a shade too thick, probably OK for 1/48th and definitely for 1/32nd scale. I’ve heard some advocates of lycra thread, but have yet to try this myself.

Blade and rod aerials on modern aircraft are often supplied with the kit, but either get knocked off during construction or are too thick and unrealistic. Fortunately they are easy to manufacture by hand from 10" plasticard or appropriately sized rod, usually available from model railway suppliers. Whip aerials can be made from fine wire or stretched sprue, whichever takes your fancy or seems most appropriate at the time. I don’t personally like etched brass replacements for rod or whip aerials, as they are just too two-dimensional, and would prefer to use slightly over-scale kit items which at least have a 3D nature (the last time I looked the universe still had three dimensions!).