If you're editing out pauses between tracks, you either need to add fade-outs and fade-ins between tracks, or you need to crossfade the tracks. Otherwise, you'll run into clicks and/or sudden changes in background noise.
For a fade-out, select the region you want to fade, and apply the "Fade out" effect; then fade in the next track the same way.
Crossfades in Audacity are a PITA, but they can be worth the effort. My technique is to do all other processing first (EQ/normalization/audience noise limiting/spot noise filtering), and then drop labels where I want tracks to start and end. I then create a second stereo track, and use Split Cut to cut out alternating sections of music, pasting them into the second track in their original position on the timeline.
Then, I use the Time Shift tool to overlap the tracks I want to crossfade, and use the Cross Fade In/Cross Fade Out effects (these are in the plugin pack, not installed by default) to do the crossfades. As with standard fades, you select the region you want to fade out or in, and apply the crossfade in or out effect.
Then I kill the extra labels. I set Audacity to snap to CD frame boundaries (preventing sector boundary errors when burning to CD) and drop new labels where I want the tracks to be split.
A side note: snapping to CD frame boundaries should only be used when you're dividing tracks to be exported. Otherwise, some editing operations might leave less-than-one-frame chunks of audio where you don't want them.