Improving actions will usually give more skill than creating actions.
You may gain skill even failing an action - that depends on ; though, it can vary per skill. For instance, [[lockpicking]] won't raise on failures, but [[carpentry]] will.
Stamina affects skill gain. If you have less than 30 stamina, you will gain half as much skill. It's recommended to have full stamina when you start an action. Action time also affects skill gain. The longer the action, the more skill you gain for that action.