Take a backup before every substantial change. Backups are easy.
I love Moodle's import and restore facilities BUT I now work in the real course (before real students are enrolled) because, very occasionally, I have had trouble with an imported (i.e. copied) course that I didn't see in the source. (These problems were usually due to slight differences in Moodle configuration or role permissions which could have been prevented by more perfect documentation and synchronisation of the test environment.)
If at all possible, I ask different people to be test-students, observers and editors. It is hard to both play the role of student and attend to details of the experience. Observers should note indicators of "flow" and photograph anything that needs fixing, for later discussion with editors. Records of the communication lead to better guidelines for design and implementation of subsequent courses.
I can't afford to test exhaustively, because my courses expect students to complete dozens of hours of interaction with lesson, quiz, forum, and peer-assessment, with some time, sequence and completion constraints. However, each time I come up with a new purpose for a particular feature, I know the first implementation needs testing and critical review.