My method of 'washing' is to add black (or dark brown) to the base colour of paint. Then, as you say, dilute it very heavily so it's almost more of a tinted thinner.
I'll run the wash over the entire model taking note of where the dark patches seem to be settling. Ideally, what you want is for the wash to settle in all the little nooks and crannies, and not so much the wide flat areas of the model. You can easily remove the wash from an area with a cloth or (lint free) tissue.
Quite often, you'll find yourself repeating the wash in certain areas until the desired effect is just right. When the wash is dry, you create a contrast by drybrushing the model with a lighter shade of the base colour. Smaller details will really stand out nicely when drybrushed.
As for the tracks, I always mix up a dark brown or dark rust colour and paint the tracks with that. After that's dry, I drybrush silver onto the parts that contact the ground, and drybrush a lighter rust colour onto other parts that would not be in constant contact with the ground while the vehicle is moving. I generally find it's not necessary to 'wash' the tracks as the base colour I use is usually dark enough.
There are lots of ways of acheiving good results. I'm sure other people will have somewhat different methods than mine. You just have to find what method works best for you.