Trigger once is usually used in conjuction with another condition. For example:
[code:382s0g7h]+If 'variable' equals 0
-Add 1 to 'variable2'[/code:382s0g7h]
If the first condition comes out true, then the action within will only be performed once. However, if the first condition later evaluates to false, then the action will trigger again as soon as the first condition is once again true, starting the cycle over.
If you want the events under the trigger once conditions to only be run at the start of the layout, it would be better to put them as sub events to the start of layout condition.
EDIT: Here's an example showing what I just described.