Hi there,
As a keen aircraft modeller and a technician in the RNZAF I can say that the whatever techniques you choose to weather your aircraft, sublety is the key.
Remember that aircraft aren't tanks, they are looked after very carefully, and although paint does get chipped, and panels and areas are re-sprayed, and hydralic fluid leaks and gun carbon sticks to it, they never look as dirty as you might think.
The reason for this is that up close, when you are working on them, or viewing them at an airshow, all the muck and damage stands out very clearly, and is pretty obvious. But stand back 50 feet and take another look, that aircraft looks a lot cleaner all of a sudden. Now look at your 1/48th scale aircraft, how close would you have to look to be as close as you would be if you were standing right next to the real thing? pretty close, using a magnifier even! At normal viewing distance of a few inches, you're model is the same size as the real aircraft at around 50 - 100 feet (someone has worked this all out, it is called 'scale distance', but the exact figures escape me).
The upshot of this is that as tempting as it is to cover your model in heavy washes, and to dry brush it to bring out all that detail, really you should stick to highlighting the panel lines with a dark wash, usually a darker colour of the base coat, but many people use thined black enamel. Some sublte paint wear around leading edges (if appropriate) and around walkways, access areas etc. A usefull technique, that needs a little practise, is if you have an airbrush, you can carefully shade around the panel lines with a slightly darker version of the base coat, and use a slightly lighter version on the upper surfaces of the the fusalage. then apply your wash, this helps duplicate the 3D effect created by light falling on the real thing.
I would suggest perfecting the wash technique before trying to master the shading one, it is quite easily over done (believe me!). I haven't really used pastels myself, but have seen very nice results on other's models, you should try and find some articles in back issues of FSM or similar magazines.
Hope this is of help.