Run the soiled area under hot water. Like any grease, the hot water will break it down and disperse it. In many cases this will remove the grease and grime from your clothing by itself.
Pour a little bit of degreaser dish soap--such as Dawn or Palmolive--onto the stain. These products have the ability to break down the grease and grime particles.
Rinse the garment in hot water. If the stain is not fully removed, then repeat the process.
Launder the garment as usual.