Use a DAW that can batch process stems. Recent updates to most major DAWs come with batch naming and export options for groups and stems. Learn how these functions work and put them to good use.
After you’ve finished a project, render all tracks to audio separately and archive them. Even if you don’t have to provide stems and cut downs right away, you might in the future. So archiving the audio means you don’t have to bother with finding instruments and plugins again.
Learn your keyboard shortcuts. You’ll whiz through your edits if you can remember important key combos for split, join and fade.
Have a few transitions ready. It’s easy to cut and paste sections of a track together with some clever overlaying of transition samples. Alternatively write your tracks with lots of short pauses, which will make them easy to rearrange in the edit.
This is properly cheating... If you write tracks at either 60 or 120 bpm you’ll be able to make short work of those 15, 30 and 60 second cut downs as they’ll fit perfectly to the speed of your track.