Quite simple really! First of all, my technique involves different type of paints at different stages so that I'm not risking big disasters... Say for instance you painted your model with an enamel base. If you do a wash straight away with an oil-based wash, you're gonna let the thinner in the wash do its 'magic' with the paint below. Believe me you do not want that!
So instead, use a water-based paint. I use artist's watercolors that comes in tubes for my washes. You can use them over acrylics and enamels or even cellulose paints and will not damage them.
Mix a little bit of the paint with water, a drop or two of washing-up liquid and a few drops of white vinegar. This very thinned down paint is a wash. You can either slap it all over (as I do!) with a wide flat brush, or be 'delicate' and apply it selectively, in recesses, with a very fine brush... If you feel the effect you want is not there yet, slap on another coat after the first has dried. If you feel your first coat is too much, place the model under the tap and let it clean itself. Then start again. After that, in my text book, comes drybrushing (that will highlight the raised areas, i.e. do the opposite of a wash), and the weathering.
If you do not want to use artist watercolors, a wash made out of acrylic paint, water (or isopropyl alcohol!), washing up liquid and vinegar will work well too. However, if you've got too much of it on, be fast in cleaning it up under the tap. Once acrylics have dried, you're gonna have a h.. of a job removing it from all those recesses!
Oil-based washes are OK too, but either ler your enamel camouflage dry a loooooong time first, or use them over acrylic base color. Simply use a bit of paint and some thinner to get the wash you need. They are slightly less forgiving than acrylic washes.
In any case, my washes are made of a color that's much darker than the base coat. I normally use a shade of dirty black/brown called Sepia. This works really well onto Olive drab, greens, German camoflages, panzer grey and the like. Pure black is too strong a contrast!
Good luck!