Step 1. Blower brush.
Step 2. LensPen SensorKlear.
Step 3. IF NECESSARY, wet swap. (Only once had to do this on my D70 and it left smears which I had to remove using the SensorKlear again)
The sensor (actually, the low pass filter in front of it) is tougher than you might think. It IS possible to damage it but it's not easy.