Once you have run all your sessions, entered the participation data and closed the sessions, you may want to mark the experiment as completed. To do so, go to the Edit basic settings page of your experiment and check the box Experiment completed?.

All this does is to move the experiment from the Current experiments list to the list of Completed experiments. That is, you may at any time 'revive' the experiment: when you uncheck the Experiment completed? box, the experiment will be moved back to the Current experiments list.