I usually use some mastering software on recordings before I encode them as MP3's. I do professional mastering work, so it's pretty easy for me to do this, but, truth be told, some nice, easy to use software exists (Ozone4, T-Racks, etc.) that make it fairly simple for the audio novice to get pretty decent results.

The main thing is to try any alter your original recording as little as possible during the compression and limiting stages, if you already have a good balance. Generally, most of those softwares will have a 'Gentle Master' preset, or something with a similar name, and these are often all you need to bring your arranger recording up to close to commercial levels (personally, I think many commercial recordings are WAY too hot for their own good, and I prefer to lose a little volume for the sake of keeping the mix honest)...

The thing is, although there are tools for editing MP3's, most of them convert the MP3 to a .wav, process it, and convert it back to the MP3 again (every encoding stage degrades the audio a little more), so to keep your audio as pristine as possible, always record as a .wav file if you can, and master THAT, then do the MP3 encoding last...

Hope this helps...