Common:
* As clean as a whistle
* As clean as a new pin
* As clean as a hound's tooth
Less Common but Still Used:
* As clean as a whistle
* As clean as fresh snow
* As clean as a surgical table
* As clean as a sterile environment
* As clean as a blank slate
* As clean as the driven snow
* As clean as my conscience (used humorously or ironically)
The best simile to use will depend on the specific context and the image you want to evoke.