I use a filter in Cool Edit Pro set to my transmitter's frequency range. The FCC doesn't allow commercial radio stations to broadcast frequencies less than 50Hz, I'm not sure what that means for us.
I run my audio through a de-esser - even the instrumentals. Cymbal crashes, hi-hats, fuzzy guitars all tend to "sizzle". I also run the audio through a compressor to help avoid extreme high and low volumes. You would be surprised how processed audio from a radio station is before it gets to your radio. The pros do it, so do I (thanks to home studio software.)
I don't know if it was thread or a different one where we shared audio tricks, however, that's my 2 cents.
Good luck!