Always pay the necessary attention to your relationship
Yes I know that some things, such as this one, should be taken for granted but unfortunately they aren't. Many couples, as time goes by, start neglecting their partners and don't spend that much time and energy on their relationship. They mainly focus on their work and don't care that much about keeping the spark alive!
Be friends, not only lovers!
Couples that have been together for quite a long time and have a healthy relationship, are more than just lovers, they're also best friends. It's very important that you can share things with your partner, trust him/her and count on him/her. Sharing your worries, your thoughts and feelings, asking for his/her advice in any occasion but also being there for him/her whenever he/she need you are what will keep you together in good times and in bad times.
Consider your relationship important but don't give all you've got.
Ok I know that what you just read worries you a bit and makes you think «what's wrong with giving all you've got? Doesn't this mean that you truly love your partner?»
Let's clear things out: Before you get yourself into a new relationship, you have your life, your friends, your hobbies, your habits. It's totally wrong to quit everything just because you found yourself a partner and depend solely on him/her. No matter how much you're enjoying your relationship, you shouldn't give up on the life you had before you got committed!