A new digital noise reduction plugin for SDRSharp has been released. It is packaged as part of the SDRSharp binaries. Head to the SDRSharp downloads page, and download and use the automated installer for rtl-sdr under the heading “Important note for RTL-SDR users”.
Then in SDRSharp, simply click “Enabled” in the Digital Noise Reduction plugin window and adjust the slider until you get good results. In testing I found significant improvements in noise reduction.