Overall sex stamina improves by healthy lifestyle. Regular sleep, exercise and meal are the foremost things.
In diet cow milk, cow butter and garlic are proven sexual tonic. Regular healthy diet containing carbohydrate, protein and minerals-vitamins is necessary. Specific diet depends on culture and geographic situation.
Exercise to control your weight and make you fit.
Sleep to relax your mind.
All above measures help you to relive mental stress also which indirectly help to perform better.
Another aspect is emotional attachment and love between partners in overall life apart from the sex. If it is good no need of any tonic, natural stamina would suffice.
Hope I have answered your query, I will be happy to help further,
regards,