One of the easiest ways to set your relationship on a downward spiral is to start taking your partner for granted. Don't ever stop showing appreciation for the things she does or the person she is, or she'll eventually start being attracted to guys who will show it. Show her you appreciate her by telling her what you love about her all the time. Thank her for the things she does, even if she does them for you all the time.
As relationships stretch into years, it can start to feel like unpaid labor when your partner takes everything you do for granted. That's why a man who shows his appreciation is super attractive to most women. Even a smile or a kiss as she picks you up from work or telling her that dinner tastes amazing can show how thankful you are.