You can easily keep your mats clean with just a few items that you will likely have around your home. A vacuum, a bucket, a garden hose, a scrubbing brush, and some soap (dish soap works just fine).
You can spray your mats first as they will likely have some mud or dirt on them. Rinse the mat as best you can and see how much of the dirt and debris comes off. If there are more stubborn patches, that's okay, as you will want a cleaner mat anyway. You can then fill a bucket with water from your garden hose and some soap to get onto the mat. Then you can begin scrubbing with a large brush to get rid of the remaining dirt. Rinse again, let it dry and you're good to go.