There is the Auto-Save feature built-in to Sketchup - Menu Window > Preferences Capture.PNG
There's also my 'Archiver' tool, that saves a new snapshot copy of the SKP into an Archive folder with the model, whenever you run it - it appears on the File menu so you can shortcut key to it and quickly save a version as you go by a simple key press... Letting you save copies at crucial changes etc as the copies are date/time stamped in order...