I've picked up a lot of the stuff below just looking at other skins or reading posts in here, particularly Citabria's and Nopoop's, as well as listening to feedback on my skins.
First, don't be afraid of using lots of layers. We aren't talking about having a weathering layer, but more like ten. Generally weathering layers should be above the paintjob and below the panel lines in the layer order.
Paint chips can be created using a light grey paint tool set to 3 pixels or so. After they are done attack them with a low opacity eraser tool. You need to make the chips vary in opacity or they look fake. This is the same with scratches, draw them with a 1 pixel tool then go over bits with a 20% eraser. The scratches will tend to go near fasteners or walkways and can be pasted onto different parts of the skin once one small patch has been done.
The paint chips need a bit more thought WRT placement. First you need to look at pictures of the real plane to get ideas. The edges of gun and engine hatches, fuel filling caps etc will get badly chipped over time. Also the areas the ground crew walk or lean on to get to them. The prop chucks stuff onto the inboard leading edges of the wings and cowling and below the aircraft too. Also aluminium aircraft have a thin skin over a skeleton of ribs and bulkheads. The paint gets worn away most where there is some structure below supporting it. This can lead to gridlike patterns of wear, particularly on wing walkways.
Oil stains around the engine cowls are common and are done like the paint chips, except in a reddish dark grey colour. Oil and exhaust streaks I do with an airbrush tool. The main thing here is not to get impatient and try to do them in one or two strokes. Use around a 10% opacity and set to about half the streak's width. Then build it up concentrating on the centre of the streak and near the origin fading out to the edges. On the exhaust use a light reddish grey with streaks of a dark redish grey.
To give the paint a dirty look create a layer which is then flood filled with either brown or grey. Usually the latter for naval aircraft. This layer is then given a sculpture effect (effects/texture effects/sculpture in PSP) to give it some variation. To make the variation less repetitive the sculpture effect is done twice, with a different size of effect on the second go. Once the layer opacity is dropped this gives a nice scruffy look to the skin. Another useful effect to go with this is the motion blur which can stretch the variation in one direction. Use this by selecting individual panels and stretching the detail in the direction of the airflow.
For rivets I use PSP's line tool. You can create lines of round grey dots, either straight or curved. Make two layers, one for horizontal lines and one for vertical. Duplicate these layers and then rub out portions of rivets to give it a worn effect. On the Havoc I also made extra rivet layers which I Gaussian blurred and turned black and white to simulate the rippling in the aluminium. Not sure if it was worth the trouble though.
Raised items like hinges and blisters are best created using low opacity airbrushes or line tools in white and black rather than lighter and darker shades of the base colour. This makes future skins much less work to create.
The way I do fabric effects is a fair bit of work. First create the framework of your control surfaces in black where they will be on the skin. Then flood fill the gaps in the framework in white and make the rest of the layer in black. You end up with a layer that is entirely black but for the white open areas in the control surface. This is used as a mask for the fabric layer. Shadows and highlights are then airbrushed and pasted onto the control surfaces while the mask stops them extending beyond the frame of the surface.
Hope this helps anyone looking to beat up there skin.
