What is the threat to your relationship? Is it internal stress? A push-pull kind of thing? Or is it boredom as some have suggested? Or maybe there's another temptation out there?
Helpful hints are:
What attracted you to each other in the first place? Is that still there? Can it be revived?
What activities did you do when you were first starting going out?
Are you still doing that? Is it something you'd like to start over again?
Often when a relationship has lasted a long time we start to take each other for granted. Do you still show your appreciation for your partner? Can you find ways of showing that more?
Another common "mistake" in relationships is to have the sole focus of your life be the other person, depending upon him/her for your every need. That may sound romantic in theory but in practice this get's very oppressive after a while. Have you been doing your own thing enough?