first thing i do is prime the subject with flat white. pre shading consists of thinning your paint very thin (i thin tamiya acrylic to about 70%thinner/30%paint) so you can work in close to the subject. turn the pressure to somewhere low but controlable for your specific airbrush (i use and omni 5000 and crank the psi down to around 10psi). use a dark color (i normally use charcoal grey to black but its really according to the finished color of your subject) and working in close outlined most if not all the panel lines using about a 1/8" line. i normally "round off" the corners and back away a little and fade the lines towards the inner panel. when this is completed, apply your top coat using a thinned down paint and layer the paint until the preshading you just did is barely noticeable. keep adding coats and "work it up" to the effect you like the best.
post shading(which is my favorite technique) is done after you apply the finished colors of your subject. apply your finish colors. then, mix a shade lighter or 2 or 3 shades lighter than your base color and shade darker or 2 or 3 shades darker than your base color. use the lighter shades for places exposed to the suns rays more than others i.e. wing tops, top of the fuselage, ect.. also apply lighter shades toward the center of the panels in a mottled technique. use the darker colors aroung bottoms of fusalage and closer to the panel lines. if done correctly these techniques can bring some very real weathering effects. do google search on preshading and postshading and it will bring up some articles packed with info on both techniques. im still in the rookie phase of this hobby and this is how i learned, through these articles. later.