Short answer: Find and eliminate the source of interference.
Long answer: Could take a whole book. This one's a bit dated but still useful:
http://www.fcc.gov/ftp/Bureaus/Mass....ook.pdf
Also, see
http://www.fcc.gov/guides/interference-defining-source
Unfortunately, unless you can get a spectrum analyzer and someone to operate it, there aren't too many other options.
Wish it were easier.