Sometimes a particle of dust can become stuck or "welded" to the sensor. The next dry step is to use a Sensor Pen or Sensor stamp. Lenspen has the SensorKlear II ($20). The SensorKlear pen has an articulating head with a sensor cleaning pad.
Dust Aid offers the Dust-Aid Platinum stamp ($30) with 12 replacement pads ($10). The Dust Aid Platinum sues specially designed pad the you lightly press onto the sensor, removing the dust. The concept is like flypaper for dust.
The following link to the Dust Aid Platinum product page also has very helpful instruction videos for using the stamp.