cawashbu
The link below has a great tutorial on the whole thing but using acrylics. I do the exact same thing but with enamels. It must be something I am doing wrong.... I just can't get the acrylic washes to work. I also use MM enamels then a couple of good coats of Future, decals then a sealing coat of Future. This is an essential step whatever method you use. I use a thin wash of MM flat black and some Testor rubber. Applied to the panel lines and let dry, then wiped away from front to back with a paper towel pad barely damp with some thinner.
http://www.swannysmodels.com/Weathering.html