Keep things romantic. Don't let your partner fantasize about a moonlit night on a tropical island with someone else because they are sure you'd never be willing to do such a thing. If you can't afford the stars on the beach, create a candlelight dinner at home, turn the lights down low and the music up high. Dress up occasionally and wear something flattering. Wear something you know your partner likes to see you in. Say "I love you", "I am so lucky I married you" and other mushy sentiments like that. They really never go out of style.
Spend time together. Sometimes long-term couples get very efficient about getting their day-to-day tasks done — and they barely see or talk to each other. If you are spending a lot of time having fun with your friends instead of your spouse, it can create big spaces that someone else could step into. Needing too much private time puts the whole idea of being a couple into question. I can't say arbitrarily what is "too little" time together, but if you start to feel unconnected from your spouse, change things, as quickly as possible.
Steer clear of temptation. There's no doubt about it: You're going to run into someone — a co-worker, a neighbor, a hairstylist, an old flame from high school — who you find unbelievably sexy and attractive. Fair enough. But if there's any risk of things getting too hot to handle, simply keep yourself out of the path of temptation. Avoid that person — and any situation that might put you in a compromising position. I advise accompanying your spouse to any kind of class reunion, which can be a major danger. I personally know of three people who left their spouse for a childhood sweetheart after reconnecting at high school reunions!
Pay attention to appearance. You don't have to be thin or look like a 30-year-old. But no matter how long you've been married, you need to watch your hygiene, avoid bad breath, comb your hair and brush your teeth and keep yourself looking as fit and healthy as possible. It even can be fun to dress up in fancy clothing every once in awhile — then to go out for a romantic evening. Attention to such matters keeps you attractive and alluring to your spouse — and helps keeps his or her eyes off someone else.
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