The off road environment is a tough one on shoes, and it pays to wash them out after every run to make them last even longer!
The best way to wash them out is to hold them by the toe and spray them with a hose as hard as you can. You'll find that all the grit and dirt that was sitting in the mesh is then washed out. Then dry them in the shade and they'll be waiting there for you ready to go the next time you are.
There are a few things that you should never do with your trail shoes. This includes putting them in the washing machine, using cleaning chemicals, using a scrubbing brush, and most importantly, never leave them in the sun covered in mud!