You can now set saveOnBlur in .tm_properties to make TextMate save files when focus is lost. Previously the recommended way was to create a command set to “Save Modified Files” with a semantic class of callback.application.did-deactivate, though incase of a save error, such command would bring up UI where saveOnBlur will ignore errors. Using.tm_properties also allows to easily target specific file types, for example one could use:

 [ ui/**.php ]
 saveOnBlur = true

This would then only have .php files in the ui folder auto-save when focus is lost.