Approaches to, not incremental saves, but finding time for a full save:

• onunload, as previously mentioned. I have found this to be reliable (but using localStorage, not network, which may change things).
• Save when your window loses focus.
• Save periodically on a timer, adjusted to when the user has not made any input recently.

Approaches to, not incremental saves, but finding time for a full save:

• onunload, as previously mentioned. I have found this to be reliable (but using localStorage, not network, which may change things).
• Save when your window loses focus.
• Save periodically: when the last save has not been in n minutes and the user has not made any input in m seconds.