Here's a couple of tips, I use them myself.
Very often a model stays looking like a model because the paint is too uniform. The trick is to put artificial highlights onto edges, and to make plastic look like metal.
There are two very useful tools to achieve this.
1. A standard graphite pencil.
2. Cotton buds.
Matt paint is good, but when you look at a real vehicle the sun still catches edges and panels.
So, using the side of the pencil lightly run it along all edges, around the doorframes, mudguards etc. Now run your finger along those same edges, or use a cotton bud. It will give a metallic look to the model.
What you can also do is gently wipe the matt surface in the middle of large panels, using cotton buds, or paper towels. This will change the texture of the surface very slightly, and give the effect of sunlight.
You can also use an old dry toothbrush to rub the edges of steel to put a slightly scratched sheen on worn metal edges.
For those that don't have an airbrush, and have found that a brush painted model has brush marks in it, there are a couple of ways of dealing with it.
1. Lightly wipe the surface over with those kitchen surface wipes that you get at the supermarket, they will take off a few microns of acrylic paint and flatten the surface down.
2. Use the brush marks to your advantage, and wipe them with weathering powder.
If I have glue residue anywhere, I often leave it, and then stipple rust effects onto it. It gives me a perfect rust texture.
When brush painting with acrylics the paint is touch dry in minutes, however, it takes a few days for all of the moisture to evaporate off. I often find that what I thought were going to unsightly brush marks completely disappear after a week.
I hope this helps, and the pic is excellent, top job!
RobG