Want more efficient MP3s - use iTunes
All MP3 encoders are not created equal.
For my money, iTunes has the most efficient encoder. You should consider using it when creating MP3s for the Web.
Here's an example. Both MP3s listed below were created from the AIF file. But while "haight.mp3" was exported from Sound Studio at 9.5 MB, "haigh.mp3" was converted in iTunes and is 6.3 MB, a third smaller.
I've had similar results for Audacity and its LAME encoder.
So my advice: Consider taking the extra step to export the file as a WAV or AIF from your editing program and import it into iTunes for encoding. Of course, you should do some comparison tests yourself to see how much, if any, improvement there is. But so far I have yet to run a test where iTunes didn't produce smaller files.