EJ - I split this off into its own thread so that you get more eyes on it. Plus, I think it's less of an Audacity problem as it is a general recording problem that can happen to anyone, regardless of software used.
I try to edit as soon after recording as possible, to minimize the differences in sound. I know this isn't always possible, though.
Mic placement is important, of course; try to place it in the same position in relation to your mouth every time.
Try recording about the same time of day, when your voice is as tired (or not) as the original recording.
Listen to a few sentences before the edit to be made, and try to match your tone, reading speed, etc. Also, don't just record and edit in the single word or phrase. Record the whole sentence (or thereabouts), and sub it all in, or cut out the phrase you need and sub in that portion. Cut and sub in at natural points, such as a breath or pause.
Other than that, you can amplify or reduce the volume on the edited part as needed to match the surrounding vocals. Usually 1-2 dB up or down is all I need.
I do all my editing (cutting, re-recording, etc.) first, then do any noise reduction, compression, and volume adjustments. That way when I notice I have a part I have to re-record, the background noise is generally the same up in my recording room, so I can just get it all in one pass at the end of my editing session.
I don't normalize. I determine what the volume is and use Amplify to get where I need it instead. Normalize doesn't give me as much control. (It adjusts the overall volume to match what the highest spike in your recording will end up at. If you have one big spike and the rest is really quiet, that will throw off the normalizing.)
I don't need to amplify, since my mic input volume and mic placement get me the volume I need. So I finish my editing (cuts, re-records), then do a quick compression (light; only -13 dB threshold), then noise removal, and export to MP3.
Noise removal vs. compression order isn't important, unless you're also making the compressor "normalize to 0 dB after compressing" (which I do not recommend, as I said what I did about normalizing). Compressor doesn't affect lower volume sound, so it doesn't affect the background noise. I would, however, do any noise removal before amplifying, because amplifying brings up the volume of the background noise, too, making more noise to clean out.
Err, guess that is about it from me. Hope this helps some!