The art of weathering is a difficult one to master... I usually weather the tracks slightly but leave the rest clean. My first attempt at weathering a full vehicle was the whitewahed Stug III at the bottom of my post. I used a whitewash method I saw in a magazine once - a coat of chalk powder mixed with water applied over the base coat. Then I used a stiff brush to wear off some of the whitewash (I probably took too much of it off though). I clearcoated to protect the whitewash then sprayed a light coat of brown on the lower hull to simulate mud. I was trying to depict a muddy vehicle in the spring where the winter whitewash was partly worn off.
The vehicles in my post from top to bottom are:
M4A3 E8 Sherman (Canadian Army, Korean war)
Sherman V Firefly (Canadian Army, WWII)
M923 and M35 trucks