Open/start resources/services only when they are strictly necessary.
Some resources require to be manually opened before use. It might be tempting to open necessary resources at the beginning of some task (e.g., upon the creation of an activity). However, resources will be actively waiting for requests, and consequently spending more energy.
Open resources only when necessary. This also avoids activating resources that will never be used.
In a mobile app for video calls, only start capturing video at the moment that it will be displayed to the user.