It surprises me that more folks painting miniatures don't base with dirt. When I want a dirt like texture, the best way to achieve that is to just use dirt. Nothing complex here at all. I just wander into the median near my home a grab a few spoonfuls of dirt. I use elmers white glue to affix the dirt, and a soft brush to clean off the models afterwards.
Sometimes I don't even paint the dirt.
Dirt: The ultimate weathering powder.