hey Shogo,
You should not have to clay your vehicle that often. Twice a year should be sufficient inclusive of one step polishing twice a year as well.
If you feel that there is grime left on your vehicle try using a foam gun so the soap dwells on the paintwork a little longer to lift the surface grime that is not imbedded in the paintwork. Be sure to be in the shade when letting the foam dwell on your paint. Then rinse the vehicle and then hit the vehicle with your two bucket method.
Just one more tip polishing and sealing/waxing is a huge key in keeping your paintwork smooth.
Hope this helps.
Cheers,
Nick