Your maximum stamina level increases with each level (only a fraction though, about 1 or 2 points).

You get 30 free stamina at 6am and 6pm. A total of 60 bonus.
You received 15 stamina in the daily online gifts
You regenerate stamina 1 point every 10 minutes

As already said, if you learn to ration it correctly, you will always have plenty.