Long-lasting relationships aren't instantly formed; they are cultivated over time. You can't instantly walk into a successful long-term relationship, but you can nurture one into existence. If you're trying to turn a good short-term relationship into a great long-term relationship, you'll need to put in effort to make it work.
Respect. First and foremost, you need to respect your partner as a human being. He/she isn't a toy, he/she isn't a tool, and he/she deserves respect. Value your partner's feelings and do everything you can to give him/her what he/she needs.