weathering is kinda an art form, you can get some pointers but the real trick is to practice. Basically a wash is super watered down or thinned paint that you kinda wipe over the spots on the model that have recessed detailing you want to bring up. In my experience its really tempting to highlight things using a very dark wash - but I have found that is usually not the best way. For something like a tomcat I'd try a gray that is only slightly darker then the base coat that you've probably already laid down. thin the heck out of it with either water or thinner depending on your paint. and kinda wipe it around, without wiping all of it off. You will be wiping 99% of it off though. The trick is not to do too much. You want just enough that you eye can pick up the detail without being attracted to it... if that makes sense. A very light drybrushing can also do a lot for real world model... a tiny bit of rust goes a long way toward making something look "real" If you know what I mean
I like to use a transparent orange for rust colors. My best advice though is to pick out a "junk model" and practice on something that you don't care if it looks like trash when its done. Scrap styrene, old broke models, whatever you find in the 99 cent bin somewhere. You can even repaint toys- you would be surprised what a decent paint-job will do for an old truck, tank, plane, tonka toy
I find that a less is more approach to weathering is usually a better approach - you can always do another wash, but its tough to fix it once you've f'd it up...Practice practice practice...
Back when I was really into model building, my technique for this was to use artist's India ink. It's as thin as and flows like water, very easy to get some on a brush and just dab it near your panel lines. The natural 'wicking' effect of liquid surface tension draws it out quite nicely, and it stays wet long enough to wipe it from anyplace you don't want it. It also ends up being a nice gray color rather than a full black since it spreads so thin. Dabbing with a paper towel allows you to compensate for areas where it may settle in a bit thicker. I think my greatest success with this technique was actually on an aircraft model, a Lockheed S-3B that I constructed for a squadron mate's father. I got many kudos for the paint job and the weathering.
As Dade said, it is a bit of an art form. It never hurts to practice a few different methods to see what you like best.
edit: Holy crap, I just read a year-old post of mine in the "Blast marks" thread where I extolled the virtues of the same technique. How very odd.
Remember that if you use a wash for your panel lines, be sure to seal your paint job and decals with Future (acrylic floor polish) and then apply the wash. Then you can hit everything with dullcoat (if it is a flat paintjob) and finish up weathering with pastels.
Yeah, Xeno is right. I should have mentioned that I use artists oils for all of my washes. I paint my figures with artists oils also, but they work great for washes and due to their rich color, really accent panel lines and nooks and crannies where shadow or dirt and grime is desired.
Oh and I believe the thinner you are looking for Xeno is called turpentine here in the states, but I just use mineral spirits with artists oils.
OK, wasbenzine, that I could find translates from Dutch to Naptha or White Gas in English. Naptha is some really strong stuff!! Mineral spirits has a lower flash point and is less vaporous. Be careful with Naptha!!