Save in manual increments every time you've done enough work so that you'll feel sad if it all disappeared.
Keep the BSP hull as simple as possible, if it doesn't block visibility it should be a func_detail.
When tracking down leaks and working on optimizations, disable the "Func_Detail" and "Props" groups in the Auto visgroups, it makes everything more manageable.
When scripting stuff, think of some naming convention, it makes things easier to keep track of.