An important part of making your miniatures look unique is to apply wear and tear. With a lot of minis, this includes things like battle damage scratches and such.
With some minis, especially tanks, I find there is a lot of depth that can be added by simply adding a little mud splattering.
Here is an example of a very mud splattered tank:
The mud splattering on the sides was created in a few steps. First, I watered down a medium and dark brown paint, and applied it like a wash to the lower portions on the tank.
When that was finished, I took the same paints and and old brush. I filled the brush with some slightly watered down paint and held it next to the mini while gently blowing through the brush onto the mini. This splatter application works quite well for blood also, and I find that the randomness of it helps out a great deal with the finished product.
One other thing to note on this tank is that the treads are painted in browns. These are the same brown paints that I used for the splatter patterns, and help unify the mini's context.
Another thing I think I should bring up is if you are using mud splattering in conjunction with a base, you should use the same colors on the base as you use with the mud:
A base helps add context to any mini, and when you are adding mud to the lower portions of a mini, it is important that the base suggests everything else you've done with the mini. In this instance, I created a base of thick mud for this tank to be crossing, and used the same colors on the base, tracks and splatter.