I normally use 'light' base colors; for me, this is actually the first step in weathering, as light base colors provide the perception of a well used vehicle, etc. (as scale size decreases - colors become lighter).
I use paint (i.e., washes, etc.) to blend and mute the various colors together.
I generally use pastels for weathering which are applied (last) over a clear flat coat (I use MMP Powders (http://www.modelmakerproducts.com
)) and then when I'm satisfied I seal same with another light clear flat coat.
Prior to weathering I look at the vehicle, gun, figure etc. and try to analyze it 'vetically' in regards to weathering. In other words, (generally, depending on the 'location' of the model and time of the year portrayed) the 'color' of the weathering will lighten as height increases.
Example: on a vehicle, working our way up from the portion that contacts the ground, you would (could have) have dried mud, then dirt, and finally road dust.
The key, I believe, is to finally blend these colors (by mixing the pastels) so that the transition does not appear too harsh. I like working with pastels as they are easy to remove and you can simply 'start over'.
Like most techniques in our hobby - they are easy to describe, but you have to practice until you are comfortable wih it.
What worked for me - 'knocked up a model quickly (still have it - and old Tamiya 1/35 Panzer II) and practiced techniques on it until I felt comfortable with the application. In fact, I still use it to try 'new' techniques that I read about.
As Jaffe Lam stated in his post in this thread - consistency is the key.