When I'm recording, I tap on the mic, wait a second or two, and reread from the beginning of the sentence. Then I just delete the flub, and I'm left with the second, clean recording.
If you want to paste something, just do it like you would with a text file. Highlight what you want, cut it (CTRL-X or edit -> cut), select where you want it to go (by placing the cursor in the right spot), and paste (CTRL-V or edit -> paste).
As for recording on a new track.. Audacity does that automatically when you stop and start a new recording. You can combine them using cut and paste ("edit -> select -> select all" or "edit -> select -> cursor to end" can help) or, if everything is already lined up, you can use "project -> quick mix".
Does that help?
Paul
