I use Turtle Wax Hybrid Solutions Ceramic Spray on my plastics. Don't spray on your seat or hand grips, it makes them slippery.
On the engine I just get the majority of the mud off then use an automotive engine de-greaser like Gunk or Oil Eater.
As far as the harness just keep an eye out for any breaks and fix them immediately. If you can, wrap your harness in split loom wrap and make sure it's taped up real good. Also use some dielectric grease in your connectors, it's fairly inexpensive and helps prevent corrosion by keeping moisture out of the plugs.